You will want a stroller whether you bring or rent will depend on you. If you are staying on site and think he will be ok walking to the parks from the bus or monorail stops and back to the room from these stops, you might want to rent in the park as needed. If money is a big issue and he still fits in his stroller, I would at least consider bringing it.
If you bring a stroller from home and it has a good storage basket, you will not need to pack quite as light. If you rent I would try to pack as light as possible. Even if potty trained however, I would stick in a change of clothes. There are spills, things to sit in and long lines for the potty!! wipes, suncreen simple non messy snacks, lots of drinks or plans to buy water in the park, cammeras, extra film, autograph book and fat pen, sun glasses for him, perhaps a mister fan for the stroller.
Just in general, don't forget you PS numbers for the day, park passes, money, lip balm, and rain gear, and and medication you think you might need. And remember that for a price, there is nothing you can't get in the park if you need it and forget to take it.
Jordan's mom