I would seriously recommend leaving this to nature and instead of
risking hormone treatments focus more on good diet and exercise.
141cm does not seem all that short for that age and I've known people
who haven't had their "growth spurts" until their mid and late teens.
Besides, you don't want to risk it going too far and have him become a
"Too-Tall-Taro" - you know, those guys that look like could snap in
half in a strong breeze :-)