[Maximo]

Dogs who eat canned food or anything other than dry food tend to drink less. They get moisture from their food.

You can encourage him to drink more by trying different water bowls (we switched to glass and my boys love it), change water sources (filtered?), and praising him every time you see him drink.

If he is constipated occasionally, then I would supplement his diet (green beans?) and make sure he is getting enough exercise.

Is the canned food a complete diet? Some canned foods are not nutritionally complete and should only be used as a topping.