Hello Hayls, and a warm welcome.
You have every right to be worried about your partner, I know I would if I was in the same situation, fortunately, I'm not, but if trust had been build before this, then you have to believe everything will be OK and that he might go out with his workmates but he knows where to draw the line.
That's easy to say but you're having these thoughts that are causing problems for you and maybe known as intrusive thoughts that pop into your head without warning, at any time causing you to believe unusual thoughts.
If I can ask you to type this, 'intrusive thoughts' in the search bar above and read the many different comments made by people who also have them, just as I do.
I'm sorry as this doesn't address your partner being o/s and if I can ask, is there any chance when he will be coming home.
Hope to hear back from you.