Hire an AILA attorney in your area asap! He or she will guide you through the process and notify the government on your behalf that you are represented and will investigate and determine the reasons for the notice. Good Luck!
I agree with my colleagues: you really need a good immigration lawyer at this point. Do you have any attorney now? If so, did they know of this issue in 1996?
It appears that he is back in the USA; how did he come back? Did he use a visitor visa, and not disclose the previous removal? Or did he sneak back in?
In any event, your husband MUST go to see ICE; it's an offer you can't refuse. But it doesn't necessarily have to end badly; a good attorney can probably work out a deal that your husband gets time to try to seek relief. Since the previous event happened so long ago, it is POSSIBLE that they could exercise their discretion and allow him to remain for the time being, because of you, and perhaps other family issues I am unaware of.
Bottom line: you have options, but they are narrowing rapidly. You will need good legal assistance. You can learn more about our firm at www.hvlawgroup.com, and we have a Bala Cynwyd/Philadelphia office.
(610) 664-6271. John Vandenberg focuses his practice solely on immigration law. You can keep up to date on immigration law by liking the firm on Facebook at www.facebook.com/hvlawgroup. His response to your question is general in nature, as not all the facts are known to him, and does not constitute legal advice. Retain an attorney experienced in immigration law to review all the facts in your case in order to receive advice specific to your case, since that protects both you and the attorney. Mr. Vandenberg's statement above does not create an attorney/client relationship.
More information is necessary. You cannot ignore ICE requests as that is the best way to wind up in detention and then removed. However, it is likely possible for an attorney to speak with them in advance of the meeting and determine a course of action that will hopefully provide time to straighten out his status and not spend time in detention.