Most are adjustable for girth. Google discount sex toys & you'll find many sites that offer dozens of varieties. Get one that's adjustable, and that's not unwieldy or awkward for sex. A thick rubber band will work almost as well.  LOL  BTW, you do have a mild case of erectile dysfunction. Lose weight, exercise, and get some ED pill too.

Cock rings have had the opposite effect on me. I suggest some ED medication doctors are usually willing to prescribe it to you, and if you have insurance even better because for me its like $22 a pill. Worth it to keep your erection from the time your cock leaves her mouth, putting on the rubber, and making penetration.

Try a leather strap with 4 or 5 snaps.  You can figure out where to snap it to keep your bone nice and hard and then when you start dropping your load you can rip the snaps open and really let the jizz flow.  Washes up nice with saddle soap if it starts getting to musky.

I found also that while rings help its not always enough. Went to my doctor found out I have type 2 diabetes, that finding out early I was able to get under control with diet and losing weight. Once the doctor knew what was wrong had no problem giving me an ED drug to try. ED can be the early signs of more major problems so seeing a doctor is always a good choice. If I had stayed with the cock ring I might not have found out about my diabetes until the problem was much worse. If its not medically related the doctor can still give you all your options but at your age I would be more concerned as to cause then the simple solutions.

Vacuum pumps that are prescribed for post op recovery after radical prostatectomy come with silicone cock rings in different sizes, also called constriction rings  They really work to prevent blood from draining out of the erection. Many also have two rings on the side for getting them off. Start by looking on the Mayo clinic web site.  

Sex shops are pretty useless in this regard.  I wanted to get a couple of back-up rings in my size, and they knew nothing about them.  Had to get them through medical supplier.  

cock rings

Have tried several types, single rings, vibrating, rope type, etc.
I find the single circular type ring works the best to keep erect and also allow ejeculation.
Try a silicon ring with steel balls imbeded in the silicon/
Partner likes the soft type with nubs at top as it does have clit stimulation, the ones with vibrator bullets are a lot of trouble and the big bullets cause discomfort on partner when hard thristing