When I was 15 I was in the same shoes as your son. So dad said, "here is the saw son, and handed me a 044". And I started my own firewood business with my younger brother.
We lived on a farm so using the tractor was a given, but I had to put diesel in it. If I needed saw gas, I bought it, same with saw chains and files.
I'd get home from school, fire up the tractor and skid tops for an hour or so, then it was time for barn chores.
On weekends I'd cut and split the weeks tow. Then as the snow got to be too deep to move around in the woods, we were selling the wood. Dad used his farm truck for this, but I had to pay him $15 for his gas, and Bro and I loaded and unloaded it.
All in all it worked out good. We could make $10 and hour if we worked at it. I used about 15 gal. of diesel and 3gal. or so of saw gas. Sold 100 face cord of firewood. Blew the money trying to fix a pos of a susuki savage that I sold to another brother for $75... But I had fun...