I just put some prostate illustrations up on my website that you might find helpful:
www.goofyfootpress.com/sexualanatomy/the_prostate.php
As for your concern about an enlarged prostate impacting your husband's fertility, that's a question that is best asked of his urologist. I can't recall seeing anything in the literature on a correlation between the two, but I haven't been looking for one, either. So I'm not a good source on this, and there are also a number of different things that could be causing your man's enlarged prostate. (If you do ask his urologist, let us know what he or she says.)
Regarding any ideas that an enlarged prostate can be due to a failure to drain the thing, I've not heard that one being bantered about in the literature. Maybe it's something that monks wrestle with, but I don't think it plagues the prostate of the average guy in the Western world.
(Regarding other kinds of "draining," there are some treatments that might involve prostate massage, but that can be a painful and unpleasant process that is best left to the urologist who considers it to be worth trying. That kind of prostate massage is very different from normal prostate stimulation that you might do as part of sexplay if the two of you enjoy it.)
As for the "aneros" device, it's a 25 cent piece of plastic that sex toy stores sell for around $30. A guy sticks it up his butt and based on the way he sits, it pushes against his prostate. I consider it to be a prostate irritant, but that's just my own personal taste.