Then work on chest activation....the mind muscle connection and all that. Even if you've only got small pec muscles compared to surrounding muscles, you can still activate them when you're working them.
Establish what cues you need in order to best work the muscle, don't just move the weight.
Drop the shoulders back and down, retract the scapula before pressing. Tense the pec in order to start a flye movement or lead with the elbow rather than leading with the hands...there's all sorts of different things you can do, just work out what works for you.
But building muscle mass on your pecs won't cure man boobs. If they're fatty then you need to drop weight. If it's gyno from puberty or whatever else then you may need surgery depending on how bad it is.