Re: Question About Building Bigger Muscles
@AprilTheSimmer96 Hmmm... I'm always trying to not get my Sims fat or bulked up. I almost always keep a treadmill on the lot as it doesn't make them bulk up. The only time I had them bulking up was when someone was using the weight machine a bunch. I didn't worry about the calorie count of the foods as they worked off what they ate fine. Personally, if I were you I would just feed him normally, but not have a treadmill.
You may want to have them do the bodybuilding career - at least for a while. If you get your Sim 'energized' they can then. Confident is also helpful. There are some good tips in this article: https://www.carls-sims-4-guide.com/careers/athlete/
Also, if you get frustrated you can do some 'sculpting' yourself in CAS, then just work to maintain it.