A.S.
Sounds to me like he's a child himself and doesn't know how to have a discussion with his girlfriend about his real thoughts and feelings.
He won't believe you if you simply tell him he's important... Or even show it for some time. Ask him why he doesn't think he's important. And don't let him give you the lame answer, "Because their dad's back... *whine, whimper, poor me*"