If you can arrange another time of year then I would. I wouldn't brave those crowds and that heat if I had another option. That being said, as others on here have indicated, you can go that time of year and have a good time. I would consider it essential to get there for rope drop, tour all morning getting the busiest rides done first, eat lunch, and head back to the hotel. Plan to take a nap, swim and rest during the worst heat of the day (also the busiest time in the park). Eat dinner at your hotel and head back to the park as it's cooling off and crowds are thinning out. Frequently an afternoon storm will pass through and cool things off before you go back to the park. You do not want to be in the parks at the hottest part of the day in July! Misery! And I am from hot, humid, South Louisiana!
Also, if your son has never done Disney, I can't see skipping it! Is there any way you could do a week at each? I know that is more expense, but Disney is an experience every kid should have, and the rides at Universal will appeal to him as well. I have an 11 y/o son and he loves Disney! This Tuesday he will get his first Universal experience!