Any update, OP? I don't know if you've figured out the details of your DH's motivation (whether it is about rejecting vegetarianism, mistaken ideas about masculinity, or just social pressures) but I, too, would be furious. I agree with others that say that energy isn't going to get you anywhere, though - I hope you've had time to talk to him on a (slightly) calmer level to figure out what's up.
I do think that taking a backseat to your vegetarianism at home is alot different from having to leave his comfort zone to support it in public. To him, it may feel like waving a banner for something he doesn't really believe in. And in a social situation, that may be really uncomfortable for him - to have to say something to other parents about it, not knowing what they might think or say, or whether other kids at the party might think it's weird and isolate DS. It's easier for someone who IS vegetarian to ask about other options, etc., because we believe in our choices, and have loads of reasons and responses for others who raise an eyebrow. But he may not have that and need help (i.e., some lines he can toss out there, or facts to back it up).
It's also FAR easier, as others have said, for a vegetarian to scan party food and concoct something resembling a balanced meal from a bunch of random appetizers. We're used to it, especially in social situations. For alot of people, if it's not a "meal" food, it won't count (hence the classic, "But...what DO you eat???" from others). Is it possible he was more afraid of not feeding DS a real "meal" than of feeding him meat? If so, he may need reassurance that you don't hate him for this one mistake, and some recognition that he was thinking about DS - but also lots of suggestions and lots of guidance on what to do in the future (including packing food).
Of course, this may not be the first time he's been in such a situation, and his decision to feed DS a hot dog may have been quite deliberate, I don't know.