Hmm, how about something involving taking a really deep breath and the inhaled air going to the muscles, something in the vein of how Donald Duck in the cartoon short Window Cleaners takes a really deep breath?

A high-school male growing during his gym training. At first he didn't notice any changes, but since growth progress, he enjoy it

Just in time for high school, I was making a story about a snow leopard growing up due to puberty! Thank you for the great idea!

Sounds great, I'm waiting for results!

I have plenty of muscle growth ideas.

1. Holding an orb-like object, the longer they hold, the bigger he gets.

2. Just growing while flexing, each flex adds mass to their certain muscle groups.

3. Growth from eating food.

4. Muscle theft. Just by touching another huge person to steal their muscles.

5. Worship growth. Getting touched om their body parts causes them to grow.

I hope some helps