At 12, I think it is probably time he was learning to manage his own money, so i would fix the amount in relation to what you expect him to buy for himself out of it. For example, you could give him £20-30 a week, but he would have to pay his own busfares, entertainment, snacks, hair cuts and comics/books, or up it a bit and he has to buy his own clothes and trainers and fund his savings for holidays and presents for others. Then he sees the choices he needs to make between designer gear and walking to school or going to the cinema with his friends. Obviously, you need to budget it in relation to what you would normally spend on him and you may want to link in the amount to what he does round the house. My kids are a bit younger (8 & 9), but we give them a fixed amount (£5) as long as they keep their own spaces tidy, but if they do extra chores, e.g bins and dishwasher etc. they get a bit extra on top (up to another £5). But, i don't buy them treats at the supermarket on a regular basis, I expect them to buy their own comics and treats and presents for their friends birthdays etc and pay for any extra things they want to do, over and above the trips and classes we already pay for.