I agree with being rather matter of fact with it, acting as if it's going to be a non event. But likely as not, it'll be a huge event for the horse & he may find trusting you to go into a trailer very hard. In that case, Lots of approach & retreat, gradually getting him confident & comfortable with small steps along the way is the best, IMO. Don't push him to do more than he's almost comfortable with, until he's fully comfortable & that comfort zone has stretched a bit further. That way, even if you run into trouble at some point, he's still really confident with prior steps, so you don't have to back up very far to overcome the rest.
I'd be doing it in lots of very short, easy sessions, with lots of negative reinforcement/stress free breaks in between. I also like to get them comfortable with the trailer as but one of many obstacles or tasks they learn, rather than focussing on it as the one important Job.
Another thing I've found is that much of the time going in is not the most scary thing, but backing out is. Therefore I also first make sure the horse is excellent at yielding to pressure & suggestion, especially backwards, and practice asking him to go in just a few steps then back, repeatedly.