I would make him a smoothie. My kids love fruits but smoothies worked. Can he sip out of a straw? This is fun too. You can put the baby food jar in there, real fruits or frozen fruits, a little milk, even a little yogurt or ice cream. A treat and nutritious. Sometimes this was her meal bc she did not eat other stuff at the time. Apple juice in there is also good- not a lot. or any juice. and water.
Would he eat them if you made the watermelons like blocks. or maybe used a cookie cutter for other fruit? I sometimes cut the apples like long strips like fries. This is fun. I don't call them fries. She just likes them this way.
Another thing that has helped me TONS-- i have a little doll or car or anything and say-- ok car, let's taste this food or banana or whatever. Then she wants it.
Don't give up but don't really force it.
Good luck.