Well, seems you guys are approaching this sensibly. AND you have properly identified the envy issue, which is always going to be lurking around the corner as circumstances change for either/both of you.
It's OK to be envious ! The key is to call it out and not let it take control. We often are going to be in positions where someone is in a better position than we are, and then we will be in a better position than them or someone else. That's just how things work.
But things change. And they especially change if we take some positive action that will move us in the direction we wish to move. It doesn't happen overnight. And sometimes we start from a position of disadvantage (him being male in this case - more often than not a disadvantage in poly potential).
But you're together in this right ? A team ?
Empathize with his position. Try to do what little extra you can while he's working towards finding/building something for himself too. And you can help in this. Nothing wrong with initiating some contact with other girls you think he'd click with - and preferably you also.
But it's harder in this area and he needs to develop patience. The worst thing is probably to go on some massive 'search' mission which will often just lead to more frustration and envy ! Just go about life, get in social situations as much as possible and see what comes your way. With a new, OPEN outlook.
No longer does he have to turn his head or shut off potential connections because he's in a relationship. The door is open for more.