Cat Meowing

I have a domestic shorthair male cat who is 16 yrs. old. He looks good for his age... walks a little slower, sleeps a bit more, but still has his spunk when we have playtime. Lately though, he's been meowing a lot. Whether he's walking through the house or lying around. Every other day or so he has loose bowels. He eats well.... science diet WD as he got sick a few years back with crystallized bladder due to the over the counter food we were feeding him. Any guesses on what might be going on with him? Thanks!