Working out is good for overall health, which is great for fertility. He should definitely be taking some vitamins.
One very common problem for men with diabetes is a low semen volume (I think this is part of our problem as my hubby has diabetes and his 'amounts' just don't seem very much). There are a few supplements that claim to help with this. We are just trying some and I'm not sure we've seen any results yet. We are trying Maca Root, L-Arginine, Omega Fatty acids, and extras of zinc, B and C.
If you talk to your ob/gyn about testing for you, you should ask for a semen analysis for him

It might at least ease your mind if all is ok!