I've been in a similar position thrice with 2 of our DS and our DD.
So what you need to do is meet the girlfriend invite her round.
and say you are both welcome at your house if you are more likely to keep an eye under your roof.
Then you can ask questions
Become a parent that can speak openly with your DS
I don't mean let anything go over your head.
But you need this girls parents name
You need to know of she is present in the home ( she could be a single working mother)
She could even be unaware her daughters have boyfriends over or she could have a handle on the situation.
The boundaries in the home have been lost.
it happens unless you enforce them in a calm manner
Like for instance you aren't nagging your son it's all about respect that works both ways.
She is 16 but they are most likely to be in the same year at school.
Always think the worst and hope for the best.
Just because he is always with her doesn't automatically mean they are having sex.
Ask him why he gave his job up don't assume it was due to having a girlfriend.
I wouldn't allow mine to stay out until midnight at that age.
But he is Mia
Only you don't have parents name or address
Speak to school if you are worried also.
You could also get in touch with early help to speak to him they are just under social services, but they are an extra influence with boundaries.
You have lost authority with your son but you can gain it back.
It's all about open communication.
good luck
And stay calm.
Teens can pull back from angry parents
I found the calm approach to work the 2nd time around
It's all a learning curve.