I started martial arts at the age of 7. There were times when I did not want to go as well. Take him to a local performance or competition if possible to show him the opportunities if he sticks to it. I was lucky enough to have a national class demonstration team at my dojo, and it performed at each belt promotion to keep the kids motivated. I eventually joined the team once I got my black belt.
Although I don't attend a dojo anymore, I still train occasionally as a part of my lifestyle. He is definitely benefitting from it, but like you said, you can't force him to do it if he's not into it anymore.