Your first stop should be a trimmed well lubed finger. Learn the come-hither motion and find his p-spot. Done during a hand-job or BJ, you should notice a huge increase in the intensity of his orgasm.
The next step for us was to use some smaller glass or metal (Njoy) toys. The smooth fitm texture makes penetration (with lube) very easy. After penetration pause to allow him to adjust to the pressure - then watching for his reactions you can begin stroking the toy in and out. This provides great p-spot stimulation for me. Starting toys in the 0.5-1" diameter range worked for me. I like the 1.25-1.5" now.
After that, there are a ton of options. Good luck and enjoy the new fun!