I was with my ex for 5 years, after 3 years he contacted his ex to 'apologise' for his behaviour. It was all behind my back until he started having a meltdown one day and I basically got it out of him that he had been texting her behind my back for a while.
We split up, I was absolutely gutted. He went back to her for a couple weeks and then realised that the reason they broke up in the first place was still there and he came crawling back.
However I messaged my ex from 8 years ago (a different guy) to apologise for my behaviour but there was no ulterior motive other than I felt shitty for the way I treated him. It was completely innocent and I just felt incredibly guilty for my behaviour way back when and once I'd apologised I felt better.
If you trust him and you can openly talk about it, I think that's the best way to deal with it before jumping to any conclusions about what 'could' be going on.