Don't some people have that reaction to Daiya? That even if they aren't allergic to the individual ingredients, they can't tolerate the combination or quantity of them?
Also food poisoning can take ages to show up- it could have been something he ate a while ago. Either spoiled food, or himself or a cook/food server not washing their hands properly. The autumn magazine for my union had an article about an environmental health officer which mentioned food poisoning.
Rosa Ellis for Unison Magazine Autumn 2016 wrote:
Recently a doctors' surgery in Stevenage needed help from Jo and the team. They said very young children were coming into the surgery with symptoms of salmonella. Jo got on the case and found out that the toddlers all attended the same nursery. After inspecting the nursery they thought they found their culprit. And it wasn't the nursery canteen. [...] The team found that the nursery had a pet tortoise. Poor Queenie was infected with salmonella, and whenever the children played with her she was shedding bacteria onto them. Luckily all the children were fine. The nursery was given guidance on keeping pets, and Queenie was treated and moved to a new home.
I told Jo and Scott I thought salmonella came from eggs and they laughed and promptly gave me a lesson in food poisoning that quashed everything I thought I knew.
Think it was that dodgy Chinese you ate last night? It wasn't. Apparently most food poisoning is caused by something you ate up to two weeks ago, and sometimes even longer. Campylobacter (the most common cause of bacterial foodborn illness) takes 10 days to gestate in your gut and it can take 30 days for you to feel the impact of listeria, a bacterium that causes a serious infection called listeriosis. Sometimes food isn't even the culprit. If you visited a farm or gone swimming in the last few weeks and you're feeling poorly you should consider that a possible cause too.
As Jill and Vixki have said, it could be a bug picked up at work, public transport, son's childcare, or any social event or public place. Hopefully he is recovered now?