Youd be surprised how easily kids fall asleep in Disney. Between all the excitement, the sun and eating more than usual, they tucker out quickly! Maybe your son wont fall asleep in a stroller at home, but its quite possible at Disney.
When my DD was almost 5, every night at dinner, right after we ordered, she's put her head on the table and pass out! I could barely get her to bed at 8pm at home. At Disney it was lights out by 5:30! I have many photos of me with a sleeping child in my arms at Disney!
When my DD was 7, we went to Illuminations at Epcot. We watched from a location near "America" where we found a bench to sit on about 30 minutes before the show. Before it even started, she fell asleep on my lap. She was probably 50 lbs, which felt like 200 when she was asleep. My husband and I watched the whole show then I asked him to carry her back to the monorail. Since we had been out all day, he was tired too, but started heading towards the front of the park. When we got close to Mexico, he had to take a break and asked me to hold her. "NO way!!!" I couldnt carry her. We tried waking her up, but it wasnt happening. We found a bench and my husband sat for a couple of minutes, then tried again to walk. He made it a few steps then was begging me to hold her! I was laughing (still am just thinking about it many years later), so obviously that made me weaker. He made it to around Spaceship Earth then the sweating and swearing began! Cursing under his breath all the way up the monorail ramp! (I thought it was the funniest thing to see him struggle! Im such a bad wife!!!) Of course we had to wait about 10 minutes to get onto a monorail, but when he finally sat he was so relieved. We were staying at the Poly so had to get off at the TTC. Down the big ramp, then up the other big ramp (no way he would walk it). We were the first stop..almost there.... but nope. He refused to get off. I got off and he made another entire loop before getting enough strength to carry her to our building, luckily right next to the GCH!
So yeah....make sure you have a stroller regardless...even if they are 15 !!!