How much pocket money should I give my son?

I pay for his food, clothes and phone bill. But he is 16 now so he needs some money which i appreciate. It would be for things like going out with friends once a fortnight and buying things we would not get for him. I was thinking �20 a week but I'm not sure. Also do you think I put condition on it? He cannot get a job as he is in full time education from 8-5 each day and on the weekends tuition and homework take up much of his time