What part do you think is bothering you?
Is it that you think he'll step over the line with an acceptable amount of screentime so that 5 years down the road you are fighting about this and you are back in the forums posting for help in determining what exactly unschooling entails and have you gone overboard into the realm of bad parenting? Read the thread about "what is the line between laziness and radical unschooling" for a preview!
Is it that you think he's squandering his money? If so, how is he getting enough to afford this on his own? Birthday money? Allowance? If it's his birthday money, is there something else you'd rather he spend it on, something you can snap a picture of and send to a happy grandma? If allowance, do you think you give him too much?
Does he have an older sibling or best friend who is influencing him? Does part of you regret introducing video games in the first place?
*I really do think you need to decide what really is bothering you and talk about it!*
Screentime? I might say that by letting him play these games that I was not wanting this to take up so much of his time, and that by subscribing you are afraid it will increase his screentime to an unacceptable level for you, and you and he will need to discuss limits.
Squandering money? Tell him the birthday money was intended for something else and grandma won't be pleased that he spent his money on this.
And if it's allowance? Tell him now he's earning so much that you expect him to start designating some of it for savings, then let him spend the rest in the way he chooses. Here's our take on allowance: allowance in our house is for learning about money (and self-control!), so chores do not earn you more money. They do what they want with their money, except we have limits on buying candy. (If it hurts when your kids squander their money, you're paying them too much.) They get one quarter for each year: DD1 gets 6, DD2 gets 4. (The Tooth Fairy gives extra allowance for each tooth.)