Sounds like you need to work at trying to give his mind a good workout. One of my dogs favourite things is when I hide a loved toy. To begin I would put him into a down stay and then just go around the corner and place the toy usually with a yummy treat on top. Then I release him and tell him to "find it" and having a big happy party when he finds it. That quickly progressed to not needing a treat and hiding in hard to find spots that can take 15 minutes to search out. He LOVES it and it helps work the mind as well as the body.
Another thing that could help would be to make him have to work for his meals a bit more. This could mean using his kibble as rewards for working on his training/tricks, things like laying a kibble track down a hallway etc. You can also stuff toys like kongs with his meals (can use canned food in addition to kibble) and freezing the night before, the dog then needs to work harder at getting his dinner.
Do lots of training. Teach lots of tricks. Consider buying some dog clothes and booties so that he can be outside a little bit. You could even expand the "find it" game to outside when possible. You can adapt human clothes lots of times to work if the dog stuff is unavailable or to save $$$.
"Never doubt that a small, group of thoughtful, committed citizens can change the world. Indeed, it is the only thing that ever has." - Margaret Mead