I am sorry to hear that your pup seems to not be his usual energetic, perky self today, difficulty getting on and off the couch and up and down the stairs.
Is he eating and drinking ok?
Is he limping, or does he seem to be walking/moving more gingerly than usual?
If he is eating ok then it sounds like he has some musculoskeletal pain that is making him feel lethargic and reluctant to do the things he usually easily does.
There are several reasons for lameness/musculoskeletal pain
Dogs that are painful will hesitate jumping or climbing stairs and display the sort of behavior he is, either because it hurts to do the activity or because they feel weaker than usual. They tend to sleep more because they are painful when they are up and moving around, and it can be exhausting guarding themselves, tensing their muscles, and having to work harder to do what used to be so much easier.
This is most commonly associated with a problem with their intervertebral discs, which are the spongy cushions between the individual vertebrae in their back and neck. These spongy discs can move or rupture and place pressure upon the spinal cord which can lead to pain, and in severe cases paralysis.
Radiographs can sometimes be diagnostic but often early on in the disease process, because the discs are soft tissue not bone, everything will look normal. An MRI is the best way of diagnosing disc disease.
If the dog is painful but has no evidence of paralysis we can try strict rest, anti-inflammatories and pain medications for several weeks to allow healing.
If there is evidence or weakness or paralysis then surgery by a board certified veterinary neurologist, as soon as possible, is indicated.
Ideally he would see his veterinarian. If this is indeed a disc problem your veterinarian can prescribe a steroid or nonsteroidal anti-inflammatory to relieve pressure on his spinal cord and nerve roots, as well as something for pain too, such as Tramadol. And if he is having painful muscle spasms then a muscle relaxant such as methocarbamol as well.
He should be closely confined starting now. No stairs, running or jumping. If you have a crate for him I highly recommend using it. The less he moves around the more comfortable he will be and the faster he will heal. He should go out on a leash to relieve himself. Do not use a collar for him, a harness which more evenly distributes forces if he pulls on his leash is better. You will need to confine him for several weeks, even as he starts to feel better or he may reinjure himself.
You can alternate warm and cold compresses on his back and neck for 10 minutes at a time several times a day. Cold reduces inflammation, and heat helps soothe painful muscle spasms.
Keeping her on the thin side is recommended to decrease stress on her back, but is no guarantee that he won't have another episode. Once a dog has one bad disc the likelihood of another is very high.
If you are interested in reading more here is a link to an excellent article about intervertebral disc disease, its causes and therapy: http://www.petwave.com/Dogs/Dog-Health-Center/Bone-Joint-Muscle-Disorders/Intervertebral-Disk-Disease/Symptoms.aspx
There are other less common causes of back pain such as infections, tumors of the vertebrae or the spinal cord itself or fibrocartilagenous emboli but far and away disc disease is the most common cause of back pain in dogs.
If he doesn't seem especially painful it may be that he has some sort of internal organ or endocrine disease that is making him feel very sluggish. Ideally he would have some screening bloodwork done, a complete blood count, biochemistry profile and thyroid profile if he seems more weak/tired then painful.
Please let me know if you have any further questions.