[Advice] about penis rings

Hey ya'll. My lover & I have been having an issue lately about the condom sliding off into my vagina. I have been looking into cock rings online, both to keep the condom on and to keep him harder for longer because condom use tends to make him on the softer side after a while. I haven't said anything to him about buying one and want to just reveal it before sex. So I am thinking that I should buy a couple different sizes just in case? Or maybe just one that isn't gonna be too tight for an "average" sized guy. Do any of you have suggestions on a basic but well-made cock ring that will help the condom stay on?