We just went in May and my youngest had just turned 5. There is a lot of walking involved especially if you're going to do one of the trails, so if he'll sit in a stroller i'd bring or rent one. My 5 yr old does not ride in a stroller anymore but was very glad to have ours there. Her and my 8 yr old took turns.
AK is one of my favorite parks! This is where we saw the most characters. Definitely go to camp minnie mickey. That was great!!!
I will also second not doing It's tough to be a bug and Dinosaur. Dinosaur was of course a little scary and ITTBAB, was fine by my kids but there were quite a few parents that had to leave with crying little ones. Some of the bugs sound very real and mean. But you know him, if he's ok with scary stuff then it was fun.
I also recommend taking the train to Rafiki's planet watch. We almost skipped this but so glad we didn't! Plus there were Jiminy cricket, pocahontas and Rafiki just hanging out with no lines in the nice cold AC!
And of course, the safari was our favorite!!!