masturbating?

Hi, I'm dating a 42 year old friend with benefit meaning were not in a full blown relationship and I've been noticing that every time after sex at night just about time to wake up for work he masturbates under the sheets. I don't know if he knows that I know but this is very strange for me sometimes I try to get to the bottom of it by asking for sex before we get up but his reply there's not enough time or if he does he doesn't ejaculate.
Why does he do this? and should I be concern to get out of this situation before it develops to a more serious relationship that may cause problems later down?

Noel's response

I would be a little cautious, but it may not be serious problem. Perhaps if you talk to a sex therapist about this they might have some words of wisdom for you.