Hi, Heather!
I love planning our family vacations! Once you decide on the timing of your magical vacation, you can then decide on all the fun stuff: resort choice, dining locations, itinerary...the fun goes on and on!
If it all possible, I would choose the September date. It is generally less crowded, shorter lines, and while the
special offers for the fall have not yet been posted, there are often some great deals, so keep checking for those. And, let's not forget how fun it is to celebrate a 9th birthday with Mickey and his pals!
The only downside of traveling in September is pulling your kids out of school, but I have found that if I give the teacher's a heads-up, they are very accommodating. The one UPSIDE of traveling in June is that because the crowds are often larger, park hours are extended, sometimes well past midnight!
My final opinion is to choose September. The weather is still a bit toasty, but June will be warm, too. For overall crowds and lines, September is the winner! Have fun planning and wish your son a super birthday!