Based on recent experiences, it may be weeks, if not months, before the police do anything. BTW this wouldn't have been the case even a few years ago, and I'm not saying it's their fault. But the police, like all public services, are having to do more with less, and in my experience (of working with this age group in schools) is that even quite serious incidents involving under 16s are not being dealt with at a reasonable speed. If you keep chasing the police, something further may happened, but it's possible they will now try and fob you off due to lack of resources.
I do think college is the way forwards here- they will be concerned about him living away from home as a 16yo in any environment, let alone one where he's living with an older girlfriend only. In reality, they would probably like him to come home, because they know his attendance/grades will likely suffer otherwise, and there could be further issues down the road. His teachers/lecturers will care because they know him, but also the college will care, as they know they will very often be the ones picking up the pieces if it all goes wrong (not police or social care increasingly).
I'd contact them again and explain he hasn't been in touch. I'd ask if they'd be willing to facilitate a meeting between the two of you. They may not be willing to do this, but they are the people with the most invested (after you) in getting him to come back home and keeping lines of communication open, so they may be willing to get involved on this level.
If they can't do this, I would still try and keep in regular contact with someone like his tutor- make sure they are aware he can always go home to you if something has gone wrong with his living situation etc.