My dog is 15 and barks constantly. He's really grouchy and we won't get rid of him, we just can't because we have had him forever. He doesn't suffer of anything so... do you know what we could do?

How is his hearing?

Has he had a good check up lately?
At 15 I find it difficult to presume that he suffers from nothing.
His eyesight and hearing may be declining, arthritis, maybe even a touch of senility.
When is he grouchy? Constantly, or when he is active, when someone maybe messes with his food bowl? First thing in the morning?
I would get him a good check up including blood panels that check organ function and see if his grouchiness is related to the fact that he is uncomfortable or sick and you just don't realize it.
Dogs tend to hide illness very well, it is natural for them to do so.
If all check out then you may just have a grouchy old man on your hands, that happens sometimes, and it's well deserved they have led a long life and just like some people they get cantankerous.

I forgot to mention he has arthritis but it isn't hurting him. His hearing is more on the weaker side but he can see okay.

If he is 15 and has arthritis is he on pain relievers? Is this why it isn't hurting him?
You can't know that he isn't hurting, dogs are very stoic and hide pain and illness.
If he is not on pain meds for arthritis then it is past time for a check up and some relief.
Has he been to the vet since this question was asked?