What exactly are your concerns? The candy that's passed out is miscellaneous "treat size" candies. M&M's, small candy bars, smarties, suckers, etc. They also pass out raisins. Some of it will contain milk and/or eggs, but not all of it.
The CS restaurants that are open seemed to carry their full menu, so that would be the same situation you'd have on any other night.
I guess I just want to make sure that there is a variety of candy available, so he can participate, and it won't be a bad experience. Skittles, suckers, laffy taffy, and other "pure sugar" candies are ok. Most if not all chocolate is not ok.
The candy is all mixed together in large barrels. The CM just grab some and put it in the party goers bags as they walk through the various candy lines.
You could do what a person I know did with her allergic child. She let the child as well as the rest of us collect candy but when we got back to her place she traded out the candy for small toys and other things that were more appropriate for her daughter to have. CAndy was divided among the adults who could have the candy.
We just let DD collect all the candy, then picked out what she couldn't eat. When she wanted to eat some in the parks we gave her options from the safe varieties. She probably got a gallon or a gallon and a half of candy, so even once we took out all the things she couldn't have - she had more than enough candy.
I'm glad that you're going to go for it - the kids had a lot of fun.