Apart from possible pain from the declawing, this may be the reason he doesn't play as much. A red flag for me is you say he "drinks a lot of water". This could be result of an all dry diet----any canned food is better than dry. But he should visit the vet as he may have some other medical condition that is not making him feel well and depressed---kidney problem, diabetes, hypothyroidism are some that come to my mind. The vet should do a complete blood workup and urinalysis. Also "sleeping all the time" is not normal behaviour for a 10 mo. old cat.
Do let us know what the outcome is. Hope everything goes well and you'll give us an update
