He should be allowed the same consideration everyone else is. If other attendees normally have the opportunity to view the menu before hand and select their meal, then by all means let him select his.
However, if the other attendees don't preselect but just pick from whatever is served, then he should also. You could order 2 fish meals (in just someone else picks the fish before he does) or put the fish aside for him (if that's what you do for the vegetarians).
POD to this. It sounds like it's option 2, though--people just pick from whatever is served, and the OP makes sure there's both meat and non-meat options. If
the chosen restaurant serves fish and if
a fish dish is in the same price range as the other foods, I might throw in a couple servings for variety. Otherwise, he can eat vegetarian for one meal, as my understanding of the term is also that fish is allowed but not mandatory.
I think I would tell him which restaurant was chosen, and that you will (as always) get both meat (including tuna salad, as the restaurant has that) and non-meat options. I would not offer to order something particular for him or even imply that this is a possibility (if you don't do this for anyone else). I would just give him the information, as you would to anyone who inquired. The only thing with greencat's wording is, I think it could be read as offering to get him something else, "if tuna salad isn't suitable." What was probably meant was, eat veggie if you don't like tuna, but he may read that as being asked, "Is tuna okay? If not, let me know and I'll find something different."