T.L.
We have one and our two boys love it. They have both used it since they were about 14 months old. It does go on the grass just not as fast. The battery will last for a few uses and it is easy to recharge. I haven't really tried it on an incline so I'm not sure about that but our backyard is pretty uneven and it goes through it without a problem. If you son is already 20 months old, he may not get as much use out of it next year. My 3 year old still used it this summer but not nearly as much. I think next year we will break down and buy a big power wheels.