I'd leave it til the day before he has to go. Too much time gives too much time to think & get anxious about it.

Talk it up. Not what you'll be doing, what he'll be doing. Maybe ask him if he can look after daddy while you're away? Give him special jobs to do for the baby while you're not there.

All my kids got a baby of their own when I had a baby. They liked to feed & change their baby etc.

GLx