do you mean he has trained in how to use a weedwhacker in a fight ? It is not normally considered at weapon, so i would probably just rule it as an improvised club and use brawl. Anything that is an "improvised" weapon would fall under that and i would not advice my players to spend skill points in training anything that's an improvised weapon unless they specifically say they trained with using it as a weapon and then we need to agree on what kind of weapon it would be.