It's different for different men, so what type of toy will depend on him.
I had to get used to her messing around back there in the begin with, so I think we started out a lot slower than most people do. First it was just her well-lubed finger during oral sex. She tried different things, rubbing, thrusting, pressing, etc., to see what I liked and get a better idea of where. Vibrations were introduced later via the Ophoria finger vibe, but I wasn't sure I liked it. We move on to the Tantus Silk Small and the VixSkin Spur, but it turned out that I did want vibrations after all. That led us to the Naughty Boy, which is what we use most often even now after we've tried several other toys.
My advice is to take as much time as he needs and learn together what works for him and what doesn't. Obviously you'll want to start with something small, but a bit of exploration might be needed before you find something that really does it for him