Responses
Testosterone burns a lot more calories than estrogen, that's why even men start gaining weight before their growth spurt (hormonal boost!!) plus when they become adults (hormones balance out and sometimes less are produced) a lot of dudes will put on weight if they're not careful. I know a lot of guys who put on a lot weight in college and had to work really hard to lose it again.
He lost weight because he got taller and became a man. Granted, not all chubby people (male or female) lose weight when they grow, but it does seem to be very common with men.