I’ll start with the bad news.
I can’t help you tell him he won’t be reimbursed because that is the wrong conversation to have. From where I am sitting and what you have shared, you need to reimburse him and then have the conversation about appropriate travel expenses going forward.
Admittedly, that probably means the tough conversation you will be having is with your boss, when you explain you need to find the budget to cover this hotel expense because you didn’t set expectations up front.
