Ginger gave you some excellent advice! A visit to the vet to rule out possible diabetes, a urinary tract infection or prostate problems should be the first thing you do, and then if those things are negative, try picking up the water bowls after dinner for a couple of nights to see if this makes a difference. As long as they are in the house with air conditioning, it won't hurt them to go overnight without water, but, of course, be sure to put a bowl down first thing each morning so that they can drink up before they start their day. It may just be that he's getting older and doesn't have the muscle tone he once had, and holding it overnight just isn't an option for him any more. If that's the case, leave the water down until about 7 pm and then let him out to relieve himself one last time before bed. If he doesn't drink any more for the night after that trip outdoors, he should be good to go until morning.
Ghilly
Has he just started doing that? Or has he been wetting at night during the whole 2 years he's come indoors to sleep?
If he's just recently started doing it there could be something wrong which might need attention. His Prostate gland could be enlarged, or he could have developed diabetes, or have a urinary infection. I don't know exactly what, but this might not be a behavioural problem, it might be a medical one.
A good idea would be to have him checked over at the vet's. Take a urine sample with you (just put a cup or something clean under him when he pees, then get it into a glass stoppered or screw cap bottle afterwards. You can do this through a funnel, or make a temporary funnel with cardboard.)
If there is something wrong, and you can get it fixed, hopefully that will solve the problem.
If you are finding it hard to exercise him, then to keep his weight at normal level, unfortunately that will mean giving him less food. It seems mean, I know, but will be good for him in the long run. Two small meals a day and only maybe one or two biscuits as treats in the day. Overweight dogs can suffer from too much pressure on their joints especially as he is getting older now.
You could also ask the vet advice about his weight. Certain medical conditions could cause him to be overweight. If not, the vet might be able to advise about diet.