Hello L,
Your son is more than welcome to attend to attend the Kid Clubs (so you can enjoy a date with your husband!). If you click
here, you can read about how Disney Cruises works with guests with disabilities. Specifically, mid-way down the page is an area about children with disabilities.
The clubs do not serve food to kids; however if children are there during meals, they'll be taken up with the group to go through the buffet line at the buffet restaurant. If needed, the kids can be taken to the restaurant before it is open to everyone.
With your son's restrictive diet, I'd make contact with the DCL Food & Beverage Team (at least a month in advance) by emailing the request to DCL.Special.Request@disney.com.
Thank you for visiting the Moms Panel! We are happy to help if you have more questions.