Any lawyer should always have an explicit written employment agreement, and any lawyer who threatens to sue over fees that are not a part of a written fee agreement is treading on very thin ice. A lawyer can only charge for fees about which the clients knows or reasonably expects to be spent and for which the client understands he is to pay. Hiring a lawyer doesn't give the lawyer an open ended carte blanche to run up as big a bill as he can and then try to coerce payment from the client. You should refer the matter to the fee dispute committee of the local bar association for resolution. Good luck.
He can sue you, but whether he is successful depends on who the small claims court judge believes. If signed a written fee agreement when you "hired" your lawyer, that will govern the outcome. If you only had an oral contract, then the outcome depends on the whether you had reached a mutual understanding. If you did not reach a mutual understanding, then the attorney will sue for the "fair value" of his services (in law it's called "Quantum Meruit"). You can call the bar and see if they have a fee dispute resolution service - some do and some do not. Since you stated that you understood that the attorney could send you a bill for for additional fees above the $3,250.00, then it appears you were aware that the $3,250.00 was not a flat fee and that you were being billed hourly. With your admission that you knew that you could be charged more (in other words you knew it was not a flat fee arrangement), I don't think your prospects of winning are good if the attorney has good time records. That being the case, I'd probably offer to split the fee with him and call it a learning lesson.
DISCLAIMER - This is general information only and not legal advice in your state. I only offer legal advice pursuant to a written fee agreement and in all cases I recommend you obtain counsel in your city. I am licensed to offer advice in Georgia and Florida only and no attorney client relationship is formed through this response.