Iirc when he overheard the other bodyguards talking about Josh's kid. He also overheard Josh turning down that one coworker who confessed, using his kid Pete as an excuse. Forgot which happened first though.
Either way, he didn't realize Pete is his own kid until the very last minute when Josh told him himself before that he's just been jealous of Pete's dad the whole time.

Can someone spoil me when does chase know that josh is an omega? (/TДT)/