In answering, I will presume that the deposit has not been returned.
GL ch. 186 sec. 15b governs security deposits. You may have a claim for treble damages, attorneys' fees and costs. Contact an attorney immediately to discuss your claim. It may be straightened out in relatively short order with a demand letter.
Your Landlord would first need to obtain a judgment against you before any attempt at attachment is made. Based on your question you may have a potent claim against your landlord for violation the Security Deposit Statute. Here's an article and resources that will no doubt help you, and without the aid of counsel.
A lot of this depends on whether there was a written agreement between yourself and the sub-tenant. Self-Help evictions are prohibited but the attorney's letter may directed to the wrong person.
Contact a landlord tenant attorney to discuss your options going forward. For a limited investment you can be informed of your rights and respond to what is presumably a 30-Day demand letter from the sub-tenant.
The most recent infestation of bedbugs all up and down the east coast has made stories like you all too common. You have several options going forward. You should contact an attorney to assure your rights are protected going forward. There would be several more questions that I would need to know the answers to before advice could be given, one way or another.
The "Landlord" need not be the owner. In order to have standing to be a Plaintiff in a summary process action you can be a property manager or proxy. However, it sounds like you're the actual Landlord. Moreover, the actual owner (your dad) doesn't want you to be evicted. Assuming you're in Quincy District there likely won't be a lawyer of the day. Spend $500 -$700 to have a lawyer enter an appearance and make a motion to dismiss. In the alternative, the much cheaper option is to physically get...
You need to speak with an experience Landlord/Tenant attorney. Oftentimes clerks will not listen to reason in this situation. You would also need to provide further details about the "agreement." Was it memorialized somehow?
Your Landlord's Notice to Quit would be considered retaliatory by the court if he commenced a summary process action against you. You should contact an attorney to go over your case. In withholding the deposit, your landlord must have complied with a lengthy, burdensome process that most landlords get tripped up in. Feel free to call my office for a free consult on the matter. I'm sure I could at least point you in the right direction to getting your deposit back.