Old boney cat

I have a wonderful old cat. He'll be 15 next February. He seems to be
thinning out getting very boney on his head, shoulders, backbone and hips.
He eats pretty well, and for the last 3 weeks I've been trying to plump him
back up so I've been feeding him canned food twice a day while still offering
his favorite dry food in his dish all day. His coat is still shiney, soft
and well groomed, and his eyes are clear and bright. He doesn't have any
weird lumps or anything else, but just seems to be too skinny. What is
happening to my cat? Is this simply a sign of old age? Is there anything
else I should be doing to improve his weight?