This just does'nt seem right...As many have mentioned it take lots of time to start showing signs of food poisoning, I find it hard to believe that he got sick on board and showed signs the same day. I have had my share of food poisoning and never felt sick the same day.
Also I doubt the 4 yr old would be the only one sick on board..
Conclusion : all signs point to him eating contaminated food the day beofre boarding.