Everybody here is absolutely right, your son is no way experiencing lag with that system setup, it's about as powerful as it gets. It could just be crappy servers which is nothing on his side. or he has a 4K monitor and simply needs to tone down the settings a ta wee bit.
Or, and I hate to bee that dude, but it's possible your son got a virus watching porn. Sorry, happens all the time to kids. What can I say.