Talk to your doctor since you are taking antirejection meds. Sometimes follow research without fully understanding what we are reading and how a combination of meds can affect you.
Take care and good luck!
As for the effect of diabetes on male fertility, there is a good website that I will list here:
http://www.medicalnewstoday.com/articles/69779.php
It does mention that diabetes can lower a male's fertility... it really depends on how good your control was all those years.
As for the drugs you mention, prednisone in particular has a side effect of lowering a male's sperm count:
http://www.spermconfirm.com/sperm-health-questions/harmful-medications
All of that being said (remember, I am not a doctor, but am answering from my own research), each person is different. And each person reacts to diabetes differently or to various medications differently. If you are concerned, you may want to ask to be referred to a fertility clinic where your sperm count can be analyzed so you know what the answer is to your chances of fathering a child.