To me based on your description so far, it sounds like he feels he is the boss and thus can reprimand the people in the house if they do things he does not want them to do.
Now, there are some medical conditions that can cause sudden aggression and those may be a factor. Unfortunately, these would not be able to be ruled out without testing. Based on the information I already have, I feel it is likely behavioral but you do need to know that there are medical causes that can contribute to a dog being aggressive.
Many dominant dogs are described as well behaved until you try to get them to do something they do not want to do, and then they reprimand you either with a growl or bite if you don't heed the growl. Things like taking away something they want, making them move when they don't want to, waking them up, etc can cause them to reprimand (bite) you. The boss dogs do the reprimanding and a dog would never dream of reprimanding a boss.
Dogs that are allowed on furniture tend to feel that since they are elevated to your level or higher if on your lap, they mentally feel elevated as well in the pack order and thus are the boss. Keeping them on the floor can help lower them mentally back to a submissive position in the pack. So the first thing is to not allow him higher that the humans or even on the same level. Keep a leash attached initially and use it to remove him from the furniture. Give a correction in the form of a quick tug and firm "NO" when he attempts to get on and a treat when he starts not trying to get on the furniture. Thus you are providing negative reinforcement for the getting on the furniture and positive reinforcement for the desired behavior (not attempting to get on the furniture). I suggest paper thin hot dog slivers as dogs tend to want to obey to get those treats while they find store bought treats boring.
There are other ways to regain the dominant position in the house as well. The best way is to start obedience training. While a formal training class is great, you can start obedience training without a formal class. The following site is helpful in helping owners train their dog. Be sure and click on the link to the page on obedience at the bottom. and links on subsequent pages leading to detailed instructions.
Training works best if you train at least 30 minutes a day (two 15 minute sessions). I would start making your dog work via the Nothing in life is free program (NILF). It is outlined below.
This training and the NILF program establishes you as the boss. Once he is listening well to the commands, have others in the household start working with him on commands as well so he sees all the people as his boss.
You will also want to keep a leash on him at all times initially to grab if he should disobey. I think you will be pleasantly surprised at how well your dog does with training. Dogs like knowing what is expected of them and they love the little paper thin slices of hotdogs that I use for treats while training. Give this a try and see how it works for you.
Additionally, I would suggest you get a basket muzzle and make him wear it anytime children are around or visitors are there. Be sure to use the leash to make him obey you. If he growls give a short tug to get his attention and a firm "NO" to let him know, you are not going to allow his aggression. If he is sleeping, give a little tug to let him know that someone is there so he is not startled when being woken up. .
In addition, if the situation is not improving using the techniques on the previous website, you may have to consult a professional behaviorist. You can usually find a behaviorist by asking your Vet for a recommendation or you may be able to find one using the following site.
Now obedience training will also help with the jumping issue. You will be able to command him to sit while visitors approach him and only if he remains calm will you allow them to pet him. Once they are seated, you can then give him a release command so he can get more affection if they wish to show it to him.
Until your dog is obedience trained, there is a method I've used for over 15 years and is very effective and not cruel for a dog that jumps on people. It cures even the most stubborn large dog. However, everyone in the family will have to be consistent until he learns it is not acceptable.
What you will be doing is putting one knee up to waist level any time you see the dog start to jump up. Put it up before the dog is close to you, so he sees it. YOU DO NOT KNEE THE DOG. Instead you put your knee up long before he reaches you and he jumps onto your knee generally hitting himself in the chest as a result. Since your knee is up and you aren't moving when it happens, he does not interpret it as something you are doing. At the same time you need to say in a low toned firm voice, NO JUMP. He'll learn that when he jumps, he ends up hitting his chest and will associate NO JUMP with that feeling and learn to not jump on people. He may still dance around on his hind legs, but they do usually learn not to touch the person. Again, I want to stress that the knee should not be used to hit the dog, but instead let the dog run into the knee.
Your dog may try and come at you from the side, but just shift position until he learns that he can't jump. You should also start teaching him that he will not get petted or get treats or affection or even talked to unless he is calm and he works for them by sitting or laying down.
Additionally they make a no jump harness that can help control his jumping until he is trained.
If he isn't neutered, you might have that done as neutered dogs tend to be less dominant usually.
I hope I've given you the answers you were seeking and excellent service as well. If so, I'd appreciate a 5 star rating. If not, or if you have any other questions at all, please reply here and I'll be happy to follow up with you.