That's a good one. You should consult a lawyer in your jurisdiction. However, I'll try and answer.
First, he wouldn't have a leg to stand on. If someone is knocking on your door, and you answer it wearing a diaper and babbling incoherently, that would not mean anything.
Second, he wouldn't be able to do much about it because you're on your own property.
Third, there's a couple of court cases that come to mind:
Slocum v. Food Fair, 100 So.2d 396 (Fla. 1958)
United States ex rel. Gerald Mayo v. Satan and His Staff, 54 F.R.D. 282 (W.D.Pa. 1971)
A. Slocum: The plaintiff sought money damages for mental suffering or emotional distress, and an ensuing heart attack plus aggravation of a pre-existing heart condition, allegedly caused by insulting language of the defendant's employee directed toward her while she was a customer in its store. The language: "If you want to know the price, you'll have to find out the best way you can * * * [b*tch] you stink to me." She asserted, in the alternative, that the language was used in a malicious or grossly reckless manner, "or with intent to inflict great mental and emotional disturbance to said plaintiff."
Slocum was tossed. It was a no-brainer. Intentional infliction and negligent infliction require behavior that is beyond the bounds of decency in a civilized society. Answering the door in a costume isn't enough to sue over. And you have a first amendment right to be obnoxious on your own property.
B. Satan and his Staff case: In that one, Gerald Mayo sued Satan and his staff for tormenting him and ruining his life, and bringing ruination to the world. A cute lawsuit, but as it turns out, the courts were prepared for this one. Since Satan could not be easily served with papers, and, unfortunately, he is considered the "Prince of Darkness", the "Prince of Lies" and principally reigns in hell, it would be very difficult to get personal service on him. In addition, his staff were unknown or unnamed, and it would be difficult to identify them and join them into the lawsuit, since they would be necessary and proper parties to suit, the suit could not progress as it would require indispensable parties to be present. The court disposed of that case, however noting that, well, Satan is a foreign potentate and the Constitution usually grants sovereign immunity to foreign potentates especially since, after all, there is comity.
All of that said, I think your JW's guy's lawyer is out of line. I suggest you speak to a lawyer in your jurisdiction, and obtain some firmer advice.
Occasionally accused of being the Devil's Advocate, and licensed in California and a number of other jurisdictions above ground and far from hell.