Remote Control Recommendations

My son and I built a few custom electric Skateboards over the past few years. Recently we finished our latest using a few parts from the older skateboard. The new one works perfectly with one exception. He is loosing connection to the remote control pretty often. We live in a Medical Center and he looses connection at several street intersections. We also confirmed the loss of connection every time he puts the remote through the open window of our car, then it gains connection once outside of the car. We did this dozens of times with consistent results. We are currently using the Maytech 2.4GHz Remote Controller MTSKR1712 Small and Light Hand Remote for Esk8. He loves the light and small remote control and would not be happy if we increased the size much. When riding in the winter he can leave his hands inside his coat and still have control of the board.

We are looking for some suggestions so that we can increase the range and maybe take advantage of frequency hopping or other technology advancements that assist in having consistent control. We have significant hills that leaves us with a safety problem when he is going downhill. If he looses control going downhill ,he will not be able to slowdown in an effective and safe manner.