I don't know about Sweden but in the US the parent needs to be the custodian for the child's behalf since a child needs to be 18 (21 in some states) for verbal and written contracts. There are tax advantages to this as well since it's taxed at the child's rate not the parents rate after 14 years old.Are you legally able to do that, or do your parents have to make the transactions for you?
My guess is his father allows him to manage this "mad money" so he learns about investing.