..from our experience, small metal rings can hurt, especially if they circle just the cock.
Correct size is really important.
Neoprene/silicone/latex rubber with proper diameter and hardness, is like Viagra with no waiting.
Cock and ball rings, being much larger, have more forgiveness. Combine them with cock ring(s), and tight fitting underwear or a boys swim suit with a hole cut for the cock, and perhaps his balls too, all of a sudden
..!
Forget about spending a lot of money for items from a sex shop or catalog. Hardware stores and parts suppliers have neoprene rings if you look hard and long enough. If you get lucky, youll find different sizes you can stack.
Vibrating cock rings ?... Ill just stick a vibrator in him
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