You are wanting to add mass to the forearms or overall ? You are pretty young mate, to be honest some form of an abbreviated workout would build general mass without wiping you out. You will still be in puberty too.
People don't normally just look at adding mass to forearms alone. I mean if you really want bigger forearms and people like mechanics and carpenters have decent forearms from the use they get when doing their job, get into that sort of thing. Otherwise just work out generally and the whole body will grow.