Well to start you off on the right track read my Work Shop on
Strangling the Cock aka Cock Rings. This will give you safety info, proper sizing and use instructions for standard and vibrating rings. You also will need to decide where he wants to wear it, around just the shaft or around the whole package. This is where proper sizing will come into play.
For a good standard ring I'll recommend the Tantus
Super Soft C-Ring, it's silicone and has some stretch so it will not be uncomfortable and fit a wide range of diameters.
For a vibrating ring there are tons of options, some will work for the woman's body some will work better for him. You may want to take some measurements to see how far up the clitoris is from his shaft. A lot of the vibrating rings have the vibe low/close to the shaft (Like the free one), for these it can require some fancy angles to get it to his the clit right. There is also rings that have bunny ears, elephant trunks etc that can help the vibes reach too.
The vibrating rings are really hit and miss but you'll never know what kinds will work until you try a few. Like Master DarkWolf is saying, finding one that has a vibe the same size as a bullet vibe may work good. Also, looking at rings with the vibe that goes up rather than across the shaft of the penis may be a good idea too.
I am sure I just added more confusion for you but I figure giving as much info as possible will enable you to make a better decision. Feel free to message me, drop a note on my wall or ask questions here. I'll be happy to help anyway I can.
