We have nerf guns and my 3yr old can pull them back probably 50% of the time now. We make him try to pull it back before we automatically help him. With time he's slowly learned to do it himself. He still needs help after he's done it himself a few times because his hand gets tired but he's getting better.
So my advice is keep having him try. Eventually he'll get it figured out. But I agree, always having to help is a pain.