A lot of the workouts for specific sports activities target the larger muscle groups without doing any of the heavy lifting you think he might want to get into, I think I would point you there towards those activities that he might then also take into High School. There are specific drills that can be done inside a gym, and outside that will give him all the resistance he needs. Just like you he might not be they type to build the visible muscles that perhaps he wants... but he will get strong just the same.
I've got 13yo and 17yo boys, both are into wrestling and football, both are naturally thin kids, and both have become incredibly strong primarily through wrestling. The older one is going to "state" championships tomorrow in 3A 145 lbs. I won't mess with him... nobody messes with wrestlers, you're on the ground and tied up in knots before you know it
Is there any particular reason that he wants to go do this with you now? Is he going into high school next year?
Both wrestling and football have built a lot of self confidence in my kids, and when the older one went into High School being immediately a part of the football team came instant "cred". If he's 14 now, he's probably going to start high school next year? If so, the high school he's going to go to will have spring camp for next years football, so that might be an area to start. My oldest was 5'7" as a freshman and weighed 135 for football and 125 for wrestling. In his senior year he weighed 165 for football and 145 for wrestling ( he worked his way down from 160, to 152, to 145 over the course of the season). He made "All Metro Linebacker" in his senior year and was capt of the team as well. You don't have to be big to achieve is what I'm trying to say.
my .02 on what has worked for us... it's a great time to be a parent, I'm having the time of my life...have fun!
eric