I am really sorry for your loss. You are obviously grieving so much and it's natural to want comfort from a boyfriend or partner.
It's so sad that they can't be there for you, but from what you said previously he did ask for the time off but it wasn't granted. Unfortunately this is what happens, it's shit but has to be endured.
So,yes, it would be unfair of you to ask him to stay when you know he can't. Please don't pile more guilt on him. He sounds a nice guy from what you have said.
Be brave - let him go off on his work trip without causing him anymore guilt than he probably feels already - and looking forward to being back with you. Don't let him go off feeling cross because you argued, feeling more guilt ridden, and not looking forward to getting back in case it's more of the same.
You will get through it and he will back to comfort you soon 💐