TG, my first question to you would be "how long has this been going on?" If it's only been a couple weeks or less, I wouldn't get too worked up just yet. I know that's easy for somebody else to say, but much like what Shamrockfaced described, I went through something similar myself about a month ago. I just couldn't maintain a full erection during intercourse. Unlike what you're describing, I was fully operative during oral/hand action, but once it was time to insert Tab A into Slot B, the best I could do was about a 75% semi. That worked fine for Mrs. Shackleford, but only up to a point: my taking forever to fully charge and (maybe) finish got miserable after a while. I was wondering what the hell was wrong with me, and of course, she's wondering if I just don't find her attractive anymore. It's an awful situation to be in.
I was pretty convinced it was performance anxiety. Our frequency of sex has been horrible the past few months for unavoidable reasons. I hypothesized that maybe I was trying too hard to make up for the lack for frequency with nothing less than mind-blowing quality. Of course, all the prep that has to go into it for us of just allows more time for me to think about it, increasing the anticipation. I did a little research, and turns out performance anxiety is actually a physiological phenomenon. The hormones that get released when you're anxious act in opposition to the vascular mechanics of an erection. It also crossed my mind that maybe something happened in previous encounter that I didn't pay much attention to at the time but it stuck in my psyche to some negative effect.
I discussed it with a couple of my friends on here of similar age, and turned out they'd all had similar issues at one point or another, most completely inexplicable and resolving themselves. While nobody had the magic bullet, I think it helped me greatly just knowing I wasn't alone or doomed.
Whatever it was worked itself out in short order and things went back to normal. Just like that, no rhyme or reason.
That said, if it's been going on for, say, a month or more with Mr. R., it is probably time for a medical intervention. I see nothing wrong with trying a pill once and seeing what happens, but as others have said, the pill might fix the boner, but there may be an underlying cause that needs to be investigated.
Honestly, I wouldn't be too quick to write off the possibility of a psychological factor, especially considering all you two have been through with your relationship in the past few months and given the issues you two had with intercourse in the past.. There could be some subconscious doubts, concerns, or pressures at play. I mean, you're back together and everything's good and all, but to some extent deep down, he may still feel like he's having to win you back and that might not happen if everything isn't perfect.