What's the housing situation? If the house is yours, I'd ask him to leave. He needs a short sharp shock.
And you say they're going on a works do soon - on the "lash"? Nah, fuck that.
You DO need to look at those messages - there's nothing wrong with this, at this point. He hasn't helped you to feel better, so it's time to put yourself first now.
Fwiw, when I thought my first H was cheating, I did look at his phone, and found the evidence. I was in self preservation mode.
With my current DH, I didn't like how much he and a colleague were messaging. Now, they weren't flirty, and it was only when they were bored on nightshift, things like sending pics of their dinner etc, but it just felt like too much messaging. I told DH how uncomfortable it made me, and he's not sent her one message since - yes, I check!
Never ever be afraid to take care of your own interests. If that means snooping, that's fine. You don't want to be blindsided when he's 10 steps ahead of you.
I hope it's nothing Op, but you do need to check those messages.