All of the previous posts were much better worded than anything I could say.
I just did want to add two things. For me, my most recent experience with nonmonogamy didn't cause problems in my relationship but most certainly shined a big 'ol spotlight on the weak spots. Also on the weak spots in my own relationship with myself. Without that spotlight I really could have gone on for a long time just spackling over the holes and ignoring problems until it all blew up and I didn't know what happened. How sad would that have been? So I look at it as an opportunity-- to work on and build the relationship so that it is so much stronger and more fulfilling. At one point I realized that my fear of losing it was getting in the way of doing some of the work. And really, if by being honest and communicating we realize it doesn't work... should we have pretended it would that much longer?
Secondly, with regard to condoms and feeling... I agree with the suggestion to do some masturbation with the condom on. I recommend using a lighter grip than perhaps normal during masturbation. One problem for many men and women is that they get used to a particular type of stimulation or particular type of pressure and without that they are unable to orgasm. The good thing is that we can all train ourselves to come in a myriad of ways, and to do that you just need to practice-- which is fun in and of itself!
Another suggestion is to put a drop or two of lube inside the condom, that seems to help a lot.
He might also want to investigate Tantra... more for the breathing than anything else. With orgasm in general, what you do with your breathing and your brain can make a big difference in whether you do or don't have orgasmsm, and the quality of those orgasms.
It might not feel "natural" at first because it's new... but just experimenting and trying things for a while you might find some new and different ways to enjoy yourselves.