Good question. If your son is making a rotating gesture for "wheels" it can be turned into the sign for WHEEL which would be using your closed fist with index finger extended on both hands and making a circling motion in lower space like at your waist level. Two wheels with arms extended outward and then move back to show the back wheels and make two circles with your index fingers on either side of your body to show the back wheels. In essence you are showing the outline of the wheels themselves.
The sign for honking a horn on a bus would be the gesture using the palm of your hand on the heel to press the imaginary horn. So yes this is similar to the sign for YOUR but the fingers would be curled up to look like the hand actually pressing at the center of the car steering wheel.
So yes your basic intuition about this is correct. You could model back to your son the correct handshape and motion for the signs to more formalize them.
If you would like more regular tutoring in ASL that could help you better help your son in his communication I would be happy to provide instruction for you.
Regards
Bill