If his sex drive is low because of medication, he needs to switch meds. When medication lowers drive, there really isn't much that can be done about it. If he isn't in the mood, no matter what you do, he most likely will still not get into the mood. Talk to him about changing his medication to one that does not affect sex drive.

He's been on them for four years... He's been trying to get off them because but he is addicted... So I dont think that will help

Sorry then

Would taking something like viagra, get him in the mood?

I have no problem getting him in the mood...

He just has trouble cumming

Well if he is addicted to these meds then he needs help. Not just with the big o but he needs help getting away from these meds. If you care about him, then I would tackle that problem first before I worried about him not being able to cum. After he gets off of the meds, he should be able to get back to normal.