Hi Donna,
How exciting, you are just hours away from an exciting Disney vacation with your family.
Stroller rentals at the Disney parks is simple and convenient. The strollers are available in either a single or double form and are hard plastic with a sun canopy over the seat. They are extremely durable and very easy to push and maneuver.
The stroller rental locations are available at the entrance to each of the parks, and when the park is first opening, they often have a number of Cast Members offering an express rental service. If you know you are going to need a stroller for your entire visit, be sure to ask the Cast Member about a length of stay rental. This allows you to pay for your entire vacation's stroller rentals on the first day. The two advantages to this is a small discount in the price over a daily rental, plus it will prevent you from having to stand in the rental line to pay, each morning. Once you pay on the first morning you will receive a voucher indicating the days on your rental. The following days you will just need to show that voucher at the stroller pick-up location to obtain your stroller. You can find additional information regarding the stroller rentals and cost
here.
On the side note, you may want to bring a towel or small blanket to cover the seat. It tends to get very hot while in the Florida sun and this will keep your grandson from being uncomfortable. Hope you have a great time!