It's pretty easy to tell an allergy, as the reaction happens within 10 mins or so of eating the culprit food. Jack has one or more food allergies, and his reaction is just as you have described, with a rash on his face that goes away within half an hour or so. Also he has had a more serious reaction which resulted in hives all over his body. So I would say there is an allergic reaction happening for your DS.

It sounds like it is going to be hard to identify the cause(s) in this case. A paediatric allergy specialist would definitely help, but they can have long waiting lists (the wait is 12 months here). In the meantime, maybe you could try a GP or paediatrician to see if they have any ideas, and I would also read up on food allergies. Try Sue Dengate's fed up with food additives site for one - Food Intolerance Network - and you can also google food allergies to find some good sites (hopefully).

It is important to find the cause(s) and avoid them, as allergies do get worse over time as the exposure increases and the reactions could become more serious. GL with it, I hpoe you find some answers.