The whole thing is making me feel a bit weird but I’m not sure why!
Well, you do know why otherwise you wouldn't have mentioned their ages and her level of attractiveness or feel weird about it.
You either think something is going on or has the potential to happen if they are away for weeks together and alone otherwise.
He sees the trip as essential to the business you have no way of knowing if this is the case.
You have no way of knowing what the accommodation/sleeping arrangements are and no way of verifying what he tells you.
You also have no way of knowing the nature of their relationship when they are working together all day and you are not there.
Why does he need two employees if the business is new and struggling for clients to this extent? I know a few people who've run web design businesses over the years and they've always worked for themselves by themselves.
It's easy for people to say you either trust him or you don't because plenty of womwn have trusted husbands/partners who've cheated on them. If you know he has a history of cheating, he's probably not a very safe bet. Otherwise, you're in the same boat as anyone in any relationship and you just hope he won't.
It seems a little odd to he going away for several weeks when only a few days of each week will be spent working and the rest will be spent doing touristy things.
The biggest concern I'd say is that, if there isnt already something going on, the conditions of the trip would make it very easy for somwthing to develop.
Not going to delete this but just read your updates...
I don’t trust him tbh he’s lied and gaslit a lot in the past, especially about trips. He’s secretive about his life and withholds a lot of information.
Dump him. This os why you feel weord abkut it. You already know he can't be trusted.
You will nott have a secure and emotionally safe future with this man because you don't have a secure and emotionally safe present with him.