If your husband is like this about the 7th grade class trip, just imagine how things will be for all the school years to come -- I predict he will be fighting many things that come along where parents are not part of the equation. He may say this about kids being in the big city "alone" but ask yourself-- is it possibly really about your husband not wanting your son to take this very grown-up step of a trip without a parent?
There should have been, as someone else noted in a post, a parent information meeting about this big a trip. It's not quite February yet, so there is every likelihood that a parent meeting is on the schedule for a little closer to the time of the trip. Get active and don't just "wait to hear" while your husband stews and your son gets upset: Check today with whatever staff member is organizing the trip, and ask if there is a meeting planned. If you're told, "Oh, we don't have that," strongly suggest that you do, and even offer to help organize it and set it up, contact parents, etc. Tell the organizer -- in confidence -- that you have a very balky husband to convince, and plenty of information is the best way to convince him. If your husband feels like this, probably a few other parents might too. A meeting and more information will help them feel more comfortable. He and other parents will relax once they have a hour-by-hour schedule of what the kids will be doing and seeing (this should be provided!) as well as full details on hotels, who the chaperones are and how much experience they have doing this trip in the past, etc.
If the school just won't hold a meeting then say you need as many details as are available at the moment given to you now so you can discuss this with your husband.
This is not the last time you will have this issue come up so fight it with facts and don't make your son the one kid who will be sitting alone, in school, doing extra schoolwork for those four days. That's what schools do with the kids who are denied trips and it singles them out.