It depends a bit on your boyfriend's age and stress levels. You need to be able to discuss this more. Your idea of coming on top is a very good one because that takes all the stress to "perform" off of him. Cialis is probably better than Viagra but better not to use either if possible. Testosterone therapy is another thought but do not push him too hard. This will come right.
Have a look at few exercises like male deer exercises, kegel exercise, sat kriya. Google these phrases and let him do these exercises. this may help, if he does these exercises for about a month. Try to know if he has started watching porno. One may lose interest in natural sex if they watch porno and masturbate.