Ditto to all the above.
Even though your hubby is a young man, one of the drugs for erectile dysfunction, such as Viagra, Levitra or Cialis may help. Have him ask his doctor for a sample. Young men tend to like Cialis best for some reason (maybe cuz it lasts for 3 days, heh heh
) ~ If his doc won't give him a sample, he should find another doctor. Beware of ordering it off the internet; you may not always get "the real thing."
Also, I would advise you to start using a condom when you are alone together, at least upon occasion. That way he will get used to it and the more familiar he is with the ritual, the better off he will be.
Sex is 99% mental so just try not to dwell on it and it will get better with time. Performance anxiety and loss of momentum is common, more common than most would like to admit. Hang in there!