As a rule of thumb, I find people expect in others the same behaviours they practice themselves. Your son's expectations of others matches his earlier expectation of himself, however, it seems he is moving on now, which makes this not so much a matter of jealousy as one of trust. Trust is hard to learn, especially when one's past experiences are not good: past betrayals make it hard to trust, others or self.
The road ahead will not be easy for him in this respect. It will require extra effort to be open, for both parties in any relationship, in order for jealousy not to be aroused while time allows for mutual trust to grow. Hopefully there will be friends to guide and encourage, where there is no room for others to doubt anyone's intentions.