That depends on a few things:
1) Safetly: the safety of materials chosen for many cock rings is questionable. The safest materials are silicone and metal, with silicone being stretchier and better for beginners. See here: link
2) Fit: Will
...
That depends on a few things:
1) Safetly: the safety of materials chosen for many cock rings is questionable. The safest materials are silicone and metal, with silicone being stretchier and better for beginners. See here: link
2) Fit: Will he be wearing it over the scrotum and penis, or just the penis? My husband has a particularly large scrotum and it's really difficult for him to get his balls through the hole in the ring, so keep that in mind when purchasing. For this type of ring, I recommend the line from Tantus (beginner, intermediate, advanced cock rings). The other type is worn only around the penis and there are a few variations in the above link.
Keep in mind his girth size and always check the "maximum stretched diameter" for the ring you are interested in to make sure he will fit, as this is sometimes a problem if the man is above or below average girth.
Also, rings that are wider (like the Tantus rings) have a decreased chance of tissue damage.
Hope this helps!