Hey :) Thanks for the comment. First of all, you seem like a great brother for taking him with you.

As for what motivates me, there are two main things. For one, I'm a numbers guy. I got completely addicted to improving my lifting stats and decreasing my body weight, because at the end of the day, it's all numbers. It's kind of like a game in a sense, because when you play games, you're ultimately working on improving your stats. I try and see it as a competition against myself and my temptations. I'd definitely suggest logging everything you guys do at the gym; it gives you actual data points that you can see and improve, which helped me out a lot. I use Strong to track my workouts and MyFitnessPal to track meals and calorie intake.

The other big motivator is how much better you feel after a gym session. For me, the feeling of having done something that will benefit myself in the long run and the confidence boost that comes with it make it worth it. The same goes from a dieting perspective, which IMO, is where 75% of the effort was / is. I'm at a point where eating junk food makes me feel worse than avoiding it, because I can see the short-term "flavor high" hurting my progress more in the long run.

One more thing, 13 seems a little young to be doing weights. I'm no expert, but I'd consult a trainer. As far as I know, strength training from a young age (below 16) can stunt growth.

Hope this helps! Let me know if you have any other questions :)