How did you get your partner to get his sperm tested?

For those of you who's partners have had their sperm tested, how did you bring it up without threatening their manhood? 
We've been TTC for 5 months and I'm wondering if my husband maybe has an issue. I've had my hormones checked and an internal ultrasound and all good. He doesn't have the highest drive in the world as he suffers from a bit of anxiety and stress from work but we usually manage to BD at least a few times during my window and I've been using OPKs and preseed and still... Nothing.. 
Suggestions please?