I know people are saying not to over plan and try to do all the things, but I would say the opposite...plan, plan, plan! Be okay if the plans change, but when we went, I had a schedule for every single day and it helped tremendously. I even scheduled our potty breaks since we had a newly potty trained child and have a lot of kids.
I forget the name of the company, but I know there is a baby sitting service - it might be "Extra Hands" - where you can hire someone to come into the park with you for a whole day. They have AP's so you don't pay for their ticket. I would do this for sure so you can ride bigger rides with your son while your little one is being cared for and entertained.
ETA: you can plan for down time/ free time in the parks - a sit down meal in AC to get off your feet...a snack in the shade in a quiet area, also...Tom Sawyer island and the Boneyard were two places that my kids loved.