Tbh I think the posts on here about coercive control and social services are a bit over the top.
I'm not saying this relationship is healthy at all, but these are relatively young teens, and as such the level of emotion involved will be high and the intensity enormous. As a parent of a teen (except when there is a serious health and safety issue) you are moving from direct management to life consultant and you need to tread carefully for fear of creating a "forbidden attraction" type scenario. It's very tedious that his mother seems to be encouraging this intensity or you could have had a private sensible chat with her about it.
How are his school grades btw? Is his academic work suffering?
In your shoes I would insist he eats and sleeps at home with you at least once or twice a week and pays proper attention to his studies. And although it's difficult currently with C-19, I would be inventing an errand you have to run in the car to collect heavy furniture which involves a couple of trips spread out over several weekends and where you need his help. (And yes I would buy furniture or car parts on eBay I didn't need if it necessary!)
During these expeditions I wouldn't refer to his girlfriend directly but I would talk about other stuff going on in his life (the music he likes,the sport he plays), and then I would talk about yourself and the healthy and non-healthy relationships you have had over the years. Explain the difference. How true friends enhance your life etc. Don't refer directly to his girlfriend.
Then ask him what he wants out of life, for his future, talk about your travel and university/college/work experiences you had after school. Talk to him about driving lessons ... . Throw in some fast food on the way back and make the trip fun , not argumentative. Do not let it get heated, keep it light.
End the trip(s) by emphasizing how much you have enjoyed his company, and how much you love him and think he is a great lad etc etc, and that you have trust in him to make good life decisions, but should he ever find that's he's in too deep with "anything" he can always use you as an excuse to extricate himself, and you are available to talk at any time.
And at some point (not on fun driving trip) have another talk with him (maybe his father could do this?) about sex and contraception and what pp said about having no control over the outcome of a potential pregnancy. (Call me old-fashioned but this situation is exactly why I don't encourage sex in this age group, not for prudish reasons, but because it's sometimes gets them in far too deep.)
Make home a fun place to be. If your ds comes back home and there are continual arguments about his relationship, you are giving him more reasons to return to his gf's house. Facilitate his sport, drive him there if you have to, (again not easy during Covid-19) but provide opportunities for him to hang out with his mates at home. Do interesting things together: let him have a go driving the car/cook the food he likes/watch the films he likes. Emphasize how much you love him and think he is great (even when you are inwardly combusting) and always be available to talk and mostly to listen (often late at night when you are just about to go to sleep yourself).
Good luck 