As a mother of an allergic child who visits his father every other weekend, these are my views:
Keep in mind that while you see the kid in alternate weekends, she is dealing with her child the rest of the time, so... you can feed him whatever you want in the weekend and she is the one who will be dealing with him for the rest of the week, the one the school will contact when child tells the teacher he feels sick after the breakfast his dad has provided, the one who will be dealing with the associated problems after the child have been eating food that he is intolerant to.
You won't notice any of that, because by the time you get to see the child in 2 weeks time, she would have been working in putting things right just for all her work to be ruined again because his father doesn't understand his actions have consequences, and in that I don't blame him, he is not around to see how what those consequences are.
Don't take the word of an 8 yr old very seriously, I know my child has told the school he is allergic to soup, and many vegetables that he is perfectly allowed to eat, but he doesn't fancy or like. Don't think either that everything the child says comes from his mum, my son has me bored sick of talk about healthy food, saturated fats, etc etc and I blame the school for that, not his father. Although I assume that if his father hear him talking, and not being aware of what DS does at school, he would place all the blame on me!
And don't take the word of your partner as the absolute truth, he might be annoyed with his ex and painting an unfair picture of her just to get your support.
And don't forget that your ex doesn't see much of his ex, therefore he doesn't have an idea of what she is up to. FWIW she might have been advised by the child's allergician to try the foods and they are getting to the end of the trial, but, if your partner doesn't communicate with her, it is easy for him to assume she has been seeing quack doctors.