Hi! Toy store employee here :)
First things first, DO NOT start with a metal ring. Ohgodpleaseno. If it turns out to be too small, it can trap too much blood inside his penis, and he won't be able to get soft enough to take it off -- you'll have to go to the ER and have it cut off. (The ring, not the penis, hopefully at least.)
Okay. Now that that disclaimer has been disclaimed, metal rings are perfectly safe once you know what you're doing and how big of a ring he needs :) and rings made out of jelly are more stretchy than rings made out of silicone are slightly more stretchy than rings made out of rubber are more stretchy than rings made out of metal, so you've got a whole list of materials to choose from. Silicone is considered to be the most body-safe of materials, especially high-end medical-grade silicone, so you can get a Tantus ring if you've got a large budget -- but for a first or second ring, to get used to the idea and to find a good size, I recommend a jelly or rubber/silicone ring at $1-$5.
Jelly rings rest at about 1-1.5" diameter, but easily stretch to much larger than any penis I've ever seen, and they tend to be the cheapest. They may also break the most easily, and they're dubiously body-safe, but again, for a first experience at $1 a pop, go ahead and grab a handful.
Silicone and rubber will rest closer to the usable size; they make them anywhere from about 2" to 3.5" diameter. I recommend starting at the "medium" size, somewhere in between those two, because: the 3.5"s are generally meant to go around the penis and testicles (if you're curious about that, just ask) so if used on only the penis, they'll likely be far too large to actually exert any kind of pressure unless your boyfriend has monster dong, and the 2" look to me like they'll be too tight on an average-sized penis; similar to trying on shirts in the store, you want to start with a size too large and go down from there, in case you misjudge. At my store at least, we also sell both the silicone and rubber in 3-packs, one of each size, so if you're really not sure, you could go with that.
As always, if you have any followup questions or would like me to take some pics of the items I have in my store, just ask :)