What Would Cause A Low Semen Volume?

My husband, age 47, has been ejaculating signficantly less semen than usual. Over the past couple of months, since I first noticed this, we've had sex 4 -5 times. Every time has been the same - he ejaculates hardly anything. Sometimes I can't even tell if he has ejaculated. When I question him about it, he says it was pleasurable and avoids talking about the specifics. One time, he lost his erection and did not ejaculate at all. I know he is not cheating on me. He's 47 years old and I think he's in good health (not on any medication). Should I be concerned? And how should I address this with him? When I attempt to address it by asking if sex felt good, etc., he says 'yes' and skirts the issue.