Hey Logan's Mom!
Welcome to the forum.
In order to best suggest upgrades for your son's computer, it would be helpful to know the rest of the specs! Namely RAM, storage (type and capacity), and graphics card.
I agree with eeee that your budget is likely rich enough to support buying new (as opposed to used) parts, but don't agree that it's unlikely your son is running a minecraft server, it's entirely possible he's hosting for his friends on his computer. I am in agreement that dropping in something like the R5 3600 would be a great upgrade for this machine - which would cost around $200. It would give him 2 more cores, each of which are faster - and a whopping 3 times the "threads", which critical for multitasking (ie - playing a game while also hosting the game's server in the background)