You could just ask him.
"I feel like you could be put off by my dating success and maybe even a little jealous. Is this how you actually feel or is my radar off?
I want to make sure I am meeting your wants, needs, and limits here and tending to our relationship even as we date others. Could you want to talk these realities over? About how to handle emotional management so we are still good and healthy as a couple as we explore this dating world? You know you can tell me things and talk to me right? I'm checking in here.
Then let him disclose or not.
You cannot FORCE him to disclose. But you could be more at peace knowing YOU are holding up your end of the stick by giving safe space and opportunity for him to disclose if he wants to.
Everyone has to hold their own baggage.