Technically, at 16, your son is old enough to be left alone but he seems like there may be things going on for him that you’re not fully aware of or aren’t picking up on.
He seems like he’s a homebody if you say he doesn’t go out much or socialise. It seems like his family and home life is something he really values, and to suddenly have that change for a man who’s just turned up and changed the dynamic must be upsetting for him.
Have you sat down and really thought about why this might be affecting him? Have you asked him like you actually care about his feelings and where they are coming from? You sound really dismissive of him in your post and like you have contempt for him getting in the way of your love life, which will be coming across to him.
I’m sure if I was like your son at 16, I might feel equally put out that mum has met a man and suddenly we don’t get to spend weekends together like before.
Is he your only son? (is it possible if so that he’s taken umbrage at the perception another man coming before him?)
Is he the youngest?
Does he have social anxiety/neurodivergence/disabilities that make him unwilling or unable to detach from you?
Did you spend weekends together before and if so, are you leaving EVERY weekend or just some?
Does he have a good relationship with his older siblings?
What does he think of your boyfriend?
These things do matter and could change things a bit.