Is it bad for your muscles to hit the heavy bag when you're sore?

[magnum]

for boxing always always do light weight and high reps. the way you are doing it builds bulk.

Hmm I probably don't need any bulk. I'll start doing more reps less weight.

But my nephew has very skinny arms and chest, not muscular at all, wouldn't it be good for him to try and build some bulk? He's pretty weak right now.

boxers who do weight lift do it very minimally. especially the smaller guys.
depending on his body type and his diet that reall determines how big he is going to get for the most part.

have him do push ups and situps and lift weights at high reps and low weight.