Watermelon is a great food to give to your son - it's soft and full of water and lycopene, which is good for him. I fed my daughter all sorts of fruits (little at a time to make sure there were no allergic reactions) at around that age, and she absolutely loves fruit. I have given her: watermelong, honeydew, bananas, grapes (cut into tiny pieces), mango, plums and apricots (cut into pieces, of course)...pretty much any kind of fruit she will eat.
However, be cautious with fruits that are acidic like strawberries and any kind of oranges/tangerines/pineapple until he is older. I gave my daughter mandarin oranges when she was 9mo and didn't even think that it would affect her, but we ended up with diarrhea for a week. Not pleasant.
The more you tempt his palette with fruits (and veggies) now, the more likely he will continue to like/eat them as he gets older. Good luck and have fun with all the different yummy fruits there are to chose from!