Good reasons to say yes:

1. You are not being a hypocrite.
2. He's not putting himself at risk of dealing with a bad supplier, if he is able to go thru your supplier.
3. If he gets in trouble about something, he'll tell you, instead of lying to you. The ability to call someone older in a bad situation without the fear of getting reamed out for it keeps kids safer.
4. Kids should gain a sense of self-efficacy early on and he's old enough to make his own decision on this one, until proven otherwise.