If he's put off telling her about you for 18 months, he's definitely not going to tell her when he arrives, is he?
I think he's planning to let things roll and see what happens. I'd question his commitment to you, as he seems to have one foot in each world.
You've shared your concerns with him and he's going in a fortnight. It's choosing time for him: he needs to be honest; you need to be strong.
I suggest you sit him down and say you're happy for him to go; no controlling, no grabbiness, but if he wants to come back again to your comfy house and his nice life with you, he needs to be honest with his friend. What about a message to tell her he is with someone and in a relationship, then he facetimes her or whatever they use, with you and you all chat, so you can be sure she understands the setup?
If he refuses this, or says he'll tell her late etc etc, I'd call BS on him. Tell him you wouldn't dream of spoiling his friendship, he's got your blessing to go and you hope they have a great time together but you wouldn't dream of letting him back into your home when he gets back.
This is him wanting to have his cake and eat it. That's not fair. You actually have all the power here. It won't take you long to go through his stuff while he's away and have it bagged up ready to go!
You don't have no right and no need to control him and his behaviour, but you have every right to control who comes into your home and lives with you and your children and you have a responsibility to make sure those people are worthwhile.