Maybe I can offer some help as well. My wife and I started tryingto have a baby after a year of being on TRT. One thing you guys have to understand, as MM said, is that all options need to be considered. Its sucks, but his sperm production MAY not ever come back, or it could be normal in 3 months. Either way, preparing yourself mentally and having many options available is the first step.
With that being said, here is my take on this. In my case I had juiced heavy for 5 years (off and on) and then came off completely. Proper post cycle therapy (pct) using only Clomid and Human Chorionic Gonadotropin (HCG) and then off everything for 6 months until the doc put me on HRT. A year later when we started trying, again, preparing ourselves, I decided to do all of my testing first to make sure I wasn't the problem, if one was to arise. Some weird results:
My sperm count was more than double what is considered normal, yet my FSH levels were VERY low. LH levels were normal, but the low FSH levels were very strange. My urologists told me that low test levels don't always have a direct correlation to low sperm counts, it just usually happens to be the case. We ended up pregnant a couple months later.
My point is, look at all your options. You've gotten some great advice in this thread. But remember this could take some time. If it were me, I would come off the Sust and do a couple rounds with Clomid, or in my opinion an even better option, Toremifine Citrate, over the next 3-6 months. Continue testing, and although he may not be as happy during the recovery stage, it should give you your best chance of success of conceiving.
Good luck.