The problem is that now that he knows about the fun you are going to need to check the vehicle for mulch debris. Of course if he is smart, he will learn to give it a quick wash - kinda like refilling the liquor bottles with a bit of water after pinching some.
I initially taught myself to drive. During the summers I worked at a golf course, err country club. One of my jobs was to lock up all of the golf cart garages. I would be finishing up and want to go home but some of the members were still in the club house drinking. So I would go into the bar and ask them if could drive their cart back to the garage for them. Most would say yeah then toss me their car keys and tell me bring their car up to club house so they didn't to walk to the garage. That great fun as I got drive a few Lincoln Continentals, Cadillacs, and a host other vehicles when I was 14.