The most likely cause of your husband's problem is a bit of performance anxiety erectile dysfunction.. He will be OK but some meds would help from time to time.
Dr Andrew Rynne.
http://www.doctorrynne.com
I found this link which could help you out. it's a checklist and at the bottom it'll given you the options or answers you may need. It's a general go by everyone is different.
http://www.webmd.com/erectile-dysfunction/guide/erection-problem-checklist
From a personal standpoint. My boyfriend kinda has the same problem, not quite like what you're describing. If something disracts him like the hpoen ringing or a knock on the door, something that requires the activity to stop he can lose his concentration and go limp. He can get it back as well. However, if whatever it is that could be distracting him, makes him go limp it could upset him enough to the point where he can't get it back up.
If he can get it back up as quickly as you say it really could just be a focusing thing or the stop of stimulation. Try putting the condom on while playing with his balls, for a lack of better wording at the moment. You could also try stimulating a hot spot on him, somewhere that really gets him going to keep him up while putting the condom on.