Grandfather? I dunno. You're 14, so he knows you jack off, but you know better than us if he'd be up for getting involved at all in that part of your life. If it was me, I wouldn't be angry or anything, but I'm not sure what I'd say about helping you with that. (I do know that if you were planting seed, I'd be sure you had plenty of condoms) At the moment, I think you should do what men before you have done for millions of years. God (?) gave you 2 hands and arms that are just the right length.
I really hope this is a real question. Typing that answer took some time...