Scarleteen

Prior to me, my partner never wore condoms. After about 15 minutes, we have to stop because he goes semi-soft (while inside of me) and complains that the condom causes a lack of circulation to his penis. He removed it and I saw that it left a ring around his penis. We use Trojans but Magnums would be too big for him. I've never had this issue with partners of similar penis size, so I'm wondering if this is something to worry about or if this is just psychological because he's not used to wearing condoms.

It sounds like it would help to try some other types of condoms - even different brands will fit a bit differently. We have a good article on different types of condoms here: http://www.scarleteen.com/article/advic ... ndom_aisle
Some people to take some time to get used to wearing condoms, but usually with some experimentation you can find something that works.