I'm in my early 40's, in reasonably good health, but have found that, when my so and I engage in long foreplay my erection gets softer, although it normally comes back when we're ready for penetration. For a while, my so had problems reaching orgasm (afraid of the noise, kids waking up etc.), and I found that I sometimes lost some hardness/fullness as time went on. Viagra (cialis less so) at 50 mg kept the erection rock hard the entire time and this helped a lot when my partner wanted to experiment with new positions etc. A couple of times with 100 mg I stayed hard enough after the first ejaculation to keep on thrusting until I came again. With the fuller, harder erection, intercourse was more pleasurable, but the biggest benefit was simply not worrying about whether I would get soft. Most times, I have sex without viagra (the cost dictates this) and it is nice to see that the equipment still works the regular way. A lot of younger guys have been using viagra to help them get erect again more quickly (especially if they ejaculate too quickly).
I hope you and your bf can wean off the pills. You may try getting him hard in non-demanding situations (oral sex, hand-jobs), and follow through until he ejaculates. This may help him associate erections with your touch and show that he can function even when viagra is not part of the picture.