Inevitable. LOL
So "Do you have a twin room" wasn't asked at any point? Or even "Oh I'm sorry it should have been two rooms, do you have another and I'll put it on my credit card"?
I remember the last civvy company I worked for, we all rocked up in Wales for a bi-monthly meeting/jolly and one of the heads of site had forgotten to book rooms for all 6 of his attendees. Funnily enough they didn't end up having an orgy and falling "instantly in love".
OK. You can't fall instantly in love. What you've got there is an infatuation and the fanny gallops.
It's pretty clear he's not interested, probably wants to get back with his partner, and is literally changing jobs to get away from you. Hurtful, I know.
Did you use a condom? If not, get tested.
Tell him that on reflection, you realise what happened was totally inappropriate between professional colleagues, especially as his direct report. From now on let's just keep it totally work related. You value him as a friend and mentor but that's all it should be.
Keep your dignity at work, have a bloody good cry at home, hopefully with a sympathetic friend who has a supply of chocolates and wine.
When/if he leaves your company, be determined not to keep in touch. Go NC. Use the non contact threads on here for support.
Good luck OP. I know it's really hard right now. Cut your losses and don't lose your dignity doing the "pick me" dance.