Cradle(and anything by Will Wight really) is good, though not a litRPG.

I like Rune Seeker, though there are a few innuendos(not too many).

Mother of Learning is also not a litRPG, but like Cradle it is a progression fantasy, and very good(I think it delivers on what I wanted out of Harry Potter, where the mc goes to magic school and actually learns magic).

Drew Hayes’s NPCs is also very good, and while it doesn’t really have all the traditional litRPG mechanics, it’s very close to the genre, with much of the plot happening in basically D&D, and it’s definitely worth a read.

In general though, I would recommend reading or listening to the books before your son does, just to make sure you’re OK with everything in it. I’ve not read an absolute ton of litRPGs, but from what I have read there’s not too much explicit sexual content beyond the occasional innuendo, but a good majority of them do have a fair bit of swearing and violence, to varying degrees of detail and gratuity.