Honestly,it looks more like where an ldr spot has scarred.

Whatever it doesn't look like any love bites I have given or received.
I get marks and bruises all the time and have noooo idea how they get there or remember getting them.
It looks quite near his collar bone does he carry a backpack or anything a lot?
Imho, I really don't think it's anything to worry about.
Although your assumption that he is up to something made me wonder if u had another reason to doubt him. If not it might be best to apologise and let it go. Especially if u plan on staying with him.
Hope u guys work it out, all the best.