So, here are a couple of things. First, if he is on prep and tested negative, he doesn't have HIV and you'd not be at risk. Of course, this requires you to trust the information he gave you. Second, how does he put your penis in his anus and you are not a willing participant in that? That doesn't really add up. Is it more likely that your penis was rubbing on his anus? That is no risk as air inactivates the virus. Third, red skin does not increase the odds of transmission and again, air inactivates the virus so the base of your penis near the ring would be exposed to air. A one time event is less than 1 percent chance of transmission. All of this adds up to only you know if you should test or not. If you actually had your penis in someone's anus without a condom, you can consider getting tested at 28 days with a DUO test knowing it will most certainly be negative.
Also I should add that before anal sex, they masturbated me and as they were wearing a ring they irritated my penis (no blood but red skin). Don't know if that changes anything. Thank you