Dear Kathy,
I'm not sure you need to do anything in particular in response to your son's interest. It sounds like he's doing fine. He might enjoy exploring the operation/construction of small engines and contraptions that operate with such engines, and pursuing such a hobby might be interesting to him. But, it wouldn't be irresponsible
simplySimply sleep to let him have this fascination and do with it what he will. Dr. KDK