There are bike seats but they are designed for toddler sizes. When my son was 4 there was no way he would fit in one. We just skipped biking that trip. I do not remember size limits for sure but I would not be surprised for you to find your son is too large to comfortably fit in the seat. Plus it is so hot and humid that he would be pretty miserable if he was crammed in there.
Google "
castaway cay bike rental child seat" and you can get an idea of the size.