Applesauces with no sugar added?

Refusing anything but water isn’t necessarily bad.

He probably wants things with a predictable mouth feel and neutral temperature that aren’t overpowering.

Hotdogs tend to be popular (the cheap ones).

If you haven’t tried cheese toast, it may be an option, although you need to be careful about the type of cheese.

Also worth trying baby carrots and seedless grapes that don’t have browned ends and are off the stem.

Something else that may be useful is having him help make some snacks; kids will often eat things they’ve made themselves when they won’t accept the same thing from someone else.

If you can afford it, you could also try a sampling party where you buy a small portions of 5 or so similar items at a time, and get him to taste them all and tell you which is the worst and best. Don’t bring “will you eat this” into it at all: it’s a game and he has to rank them. In order to rank them he has to taste them.