I would clip them and see if it makes it any better! Keep his coat in the winter so he stays warm, but it’s probably better to clip him in the summer so he doesn’t overheat. Like the others said, you should try and get him on an exercise schedule so he can get more fit. Hope this helps!
You can full clip a horse in the summer, but the sweating should go down the more fit they get!!
It depends of the period, right now is not a very good moment to clip your horse but i think in winter it will be necessary if you want to work him.
Maybe put him on a little diet, if he's less overweight, he will sweat less :)