Is he able to file a claim while the bankruptcy is going on?

He can file a claim but he might not get more than a small percentage of his claim. You are free, of course, to object to the claim, Retain an experienced bankruptcy lawyer; it's worth it.

You should have listed the landlord on the schedule of creditors. In fact, you said under oath at the meeting of creditors that you listed all your creditors. If your landlord gave you a verbal assurance that you did not owe him any money, I suppose you could have believed he wasn't a creditor. But now you have to get him added to the list of creditors so he can file a proof of claim with the court. It shouldn't be too hard to do. Once he's listed as a creditor, he will have to file a proof of claim if he expects to participate in any distribution from the bankruptcy.

Remember that part where you swore an oath that you had listed all your assets and all your debts? There isn't a bit that says "you can pick and choose which debts you list". Didn't your lawyer tell you something along the lines of "List everybody you might owe because there is no problem with listing people without a debt but it can be a big problem to miss somebody.| The first issue is whether the landlord knows of your bankruptcy despite not getting a notice from the court. Did you tell the landlord that you filed? If so, did you tell him before the cut off date for filing claims. Then the debt to the landlord is included in the bankruptcy, the problem is proving it. Did you tell him in writing, in an email etc? If not, then you might come to a "he said/she said" in court - which is expensive and the outcome is uncertain. If the landlord didn't have notice of the bankruptcy, can you hold him to a verbal change in your contract. Sure, the contract probably says all changes must be in writing but over the course of dealing with him did you develop a practice of making verbal changes? You can usually enforce a verbal agreement, if it has become habitual in your relationship. Also, did you change your position based on the verbal change in the contract - arguably you changed your position by not listing him as a creditor in your bankruptcy - because you believed the deal was valid and the lease was terminated.

Claims have to be filed in a timely fashion. Ask your lawyer to review the timeliness of the claim.

This obligation should have been schedules. If the case is still open, you need to amend your schedules to show this claim and serve the landlord with notice of the bankruptcy. If you received your discharge already, you could move to reopen to address this pre-petition claim.