You didn't say if he has seen a speech pathologist. If he hasn't then he should. A speech pathologist will be able to show him exercises that may be very helpful both for opening his mouth wider and for swallowing.

My other suggestion is to encourage him to try different foods. Everybody is a little different. Some of the foods that were supposed to be easy to swallow weren't easy for me. When pudding just kept sliding around in my mouth going nowhere I had no trouble swallowing poached eggs. As a matter of fact I was surprised that you say he can drink water because water was actually the very last thing that I could swallow without choking. It went everywhere but where it was supposed to.

My doctor telling me that I "should have no problem" did not do me any good when I was choking. I was totally scared out of my mind. That was another thing that the speech pathologist helped me with.