Thanks for the additional information. I really appreciate it.
It sounds to me like now that the baby is a bit older (and likely starting to be mobile), Dennis is feeling a bit out of place...which is normal, but needs to be managed so that this aggression doesn't get worse.
So if this were me, the first thing I'd do would be to get him involved in a basic obedience class. Not because he needs to learn to sit and stay (although that's awesome too), but because the class will help form a stronger bond between you and him and will elevate you into the position of top dog in the house because you'll be asking certain behaviors from him and expecting him to follow through.
The next thing I'd do is to immediately implement the NILIF (Nothing In Life Is Free) program in your house. In my opinion, the NILIF program offers the best payoff for most dogs and their owners. With NILIF, your dog complies with your commands and you do not need to bully or use physical force (such as 'telling him off'). The idea is that you bolster your leadership and cultivate your dog's respect for you by controlling all his resources. Specifically, you determine when you put his food bowl down and when you pick it up. You set the time for playing with toys and when that game ends. You initiate grooming and petting sessions.
By controlling his resources, you elevate your status in the eyes of your dog. I particularly like this method of training because it works on a wide range of canine personalities, including shy, easily distracted, high energy, and pushy dogs. Shy dogs gain confidence, distracted dogs develop focus and patience, pushy dogs learn manners.
Here's how NILIF works: start by giving your dog the cold shoulder when he demands your attention. Ignore him if he paws at your hand, barks at you or brings a toy to get you to pet or play with him. Don't utter a single syllable or push him away. Just act as if he is invisible. This is not meant to be rude or cruel. Rather, you are training him to understand that he cannot demand your attention any time he desires. The light bulb will turn on in his brain as he realizes that it is you, not he, who calls the shots in the house.
NOTE: Be prepared for an increase in unwanted behavior as you implement your new strategy. Your dog is going to try even harder at first, probably misbehaving even more things, since his tactics worked in the past. Do not give in!!
All members of the family must participate in the new house rules. Let them know that from now on, your dog must earn his paycheck (praise, treats, playtime) with proper behavior. At mealtime, ask him to sit and wait before you put the bowl down. When you want to play one of his favorite games, such as fetching that tennis ball, tell him to lie down before you toss the ball again. When you are done with the game, tell him game over, pick up the pall, and put it out of his reach. Do this calmly and walk away. The key to success is being consistent. Every time you want to toss your dog a small treat, have him do something such as sit or do a trick, before you hand over the tasty morsel. When you approach the front door to walk him, make sure he knows that you always exit and enter doors before him. At your dog training class, your dog must do what you've asked before he gets a treat.
The bot***** *****ne is that NILIF establishes a clear ranking in the household with the adult humans in the number one spot. It is done without meanness or punishment, but rather as a simple fact of life. In time, your dog will stop doing anything that would be considered an insult to a pack leader...such as challenging them for authority or trying to guard anything that doesn't belong to him.
This is not an easy road or a simple fix for a problem. Getting your dog back in line is going to take time, dedication and commitment from everyone in the house, but I have no doubt that with some patience and some training classes, you'll be able to get this behavior stopped.
I hope this helps.