I almost hoped it would be as easy as saying 'neuter him', but you're already doing a good job with this and your roommate too! Images available only to Customers.
The next suggestion would be to take extra steps to keep him inside for at least the next month (it won't be easy), hoping to break the wanderlust.
If you have a patio, backyard or any other relatively safe outside area that you can control his wanderings in, have him out there while you're supervising and bring him in when you come in.
You can train him to accept a harness (rather than just a collar) and with a lot of patience, even adjust him to a leash. Unlike a dog; however, a cat should never be left unsupervised on a leash, chain or tether. Never.
It's also important that he always be wearing a collar with an identity tag that has a current phone number, or have him microchipped (we do both with all the cats in our care, 'just in case' one ever escapes!).
It would be worth the added vigilence to keep him safe from neighborhood dangers. Cats are extremely independent and strong minded, but they can be taught and they can learn 'compromise' (I'm not sure we'll ever totally tame them). If he learns that he can only go out a certain door (to a safe area like a back yard or patio), and that he cannot leave that area, it might be a more acceptable option than not going out at all.
We have an older guy who, in his younger years was quite the wanderer in an isolated neighborhood without traffic or dogs. He's learned that he can only go out one door at this home and no further than the fenced area. We've got sprinklers at the edges - and when he first began to wander too far, we'd turn on the sprinklers and open the patio door. He would bolt right back into the house. We only had to do this a few times before he stopped approaching the perimeter. Now he just goes out onto the patio and into the acceptable areas nearby, but no further. Of course someone is always out there with him, keeping an eye on him and ready to turn those sprinklers on Images available only to Customers.
You can do this! Just be determined !
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Many have forgotten this truth, but you must not forget it.
You remain responsible, forever, for what you have tamed.