It's not unusual for partners to ask for reassurance of their importance early in the poly experience. It's scary to think that the person you love may find someone "better".
In my opinion, as long as you and he continue to communicate and you meet his need for reassurance, there's nothing to say that he won't become more comfortable with your relationship with others.
Me: 48 - Married, straight, male
Shiela: My wife.
Suzanne: My FWB
Adam: Shiela's LDR