I work with special needs teenagers and know how stubborn they can be. And how hard they'll work to get out of doing chores lol. However, I must say, if your son has a car and is buying gas for it, presumably he's cognitively able enough to drive. In that case he's able to process instructions for chores.
Have you spoken to your ex about this? When parents live apart it'd be an unusual teenager (high ability or one with special needs) who didn't try and pull a fast one from time to time. I've seen this in my own family, and with my pupils with divorced parents.