I have a theory on how he saved the money, if he didn't plan it for over a year.
When I was in high school, I got my first job. Of which I caught the bus to or got dropped off. I saved my paychecks for about a year to have enough money to buy a car. This cost me $2,500 and I made $10 as a part time worker.
So the shooter didn't have a car or drivers license. It could be he was saving his paychecks for a car, but then decided to go active shooter instead and spend that thousands of $ on guns and ammo instead. So it is possible if his grandparents didn't give him money or have family money, that he could have had enough to buy the guns.
But on the other side, I have friends who were like me, but didn't have to buy their own car. So they didn't have any $ saved up because there was no goal to reach. So paycheck was spent fully as they earned it.