Why not give your child a couple of dollars to spend on anything he wants and then start a savings account for him? Let him help you make charitable donations when you make them. That way he can have a couple of dollars to spend and not have to sit down and do math and think about all that other stuff.
He's 5. Not really old enough to understand he got $5 and only gets to spend $2.
I do think you have good intentions but he's rather young for all that.
His allowance is money he can burn in the fireplace, spend on bubble gum, buy Hot Wheels, give away, and you don't get to ask him what he bought with it and you can't say no to anything he wants because "you should have spent your allowance on that".
Allowance is money that he doesn't have to be accountable to anyone for. So at this age it should be small but big enough he can buy a candy bar or something small. A gift of money. Not something he should be accountable for in any way.