Rhiannon - I think Robinw's right, get a copy of E for Additives asap! Azo dyes are E102-130 approx, I think. But flavourings and preservatives can be pretty bad too - I'd cut out all additives and gradually reintroduce them. But it may be further complicated if he's reacting to some other food as well, eg wheat or egg. In this case, you would see an immediate improvement when you go off the additives, then after 3 or 4 weeks, the other reaction would kick in. Of course, by then, you might be reintroducing E numbers, so you'd think it was a reaction to them...it is quite hard and time consuming, but definitely worthwhile to get it right.
by the way, a further problem is that additives can be hidden inside other ingredients, eg "lemon juice" probably contains a preservative.
As a general rule, avoid all processed foods. This means none of the following:
longlife bread
shop cakes, even fresh from the bakery (but see below)
biscuits (see below). The ingredients always look OK, but they've always got something hidden in them.
ready made puddings eg Angel Delight
cereals (see below)
margerines & spreads (see below)
processed cheese, eg cheese triangles
most yoghurts and fromage frais
some tinned fruit has colours or preservatives in.
processed meat eg ham, bacon, sausages (see below)
processed frozen food eg fish finger, potato waffles
ice cream
sweets (see below)
bought pizzas
ready made pasta sauces, chicken sauces etc
jam (see below)
fruit juice - no pineapple, longlife apple or any other juice drink (see below)
fruit squash (see below)
and anything else processed you can imagine!
Foods which are OK include:
fresh bread from the bakery
biscuits: all butter shortbread where the ingredients don't include any funnies. Jordan's Fruesli raisin and almond gives me a reaction.
some Kellogs cereals are ok, eg corn flakes, rice crispies. Honey puffs are ok.
PURE spread is ok, so is Granose (health food shops)
some yoghurts/ff don't contain flavourings or colourings.
there are some sweets available at health food shops that don't contain additives.
Non "processed" cheese is ok of course, if it doesn't have a colouring in.
Bonne Maman Blackcurrant jam and Coop Blackcurrant preserve are the only two commonly available that don't contain citric acid. Wilkinson Old times marmalade is ok too, and one or two of their other types of marmalade.
Cakes are tricky - like biscuits they can have stuff hidden in them. Some expensive supermarket ones look ok though, as do some from health food shops.
Pure fruit juice - orange, freshly pressed apple are OK. Others are not.
Squash - organic additive free orange squash is ok.
Another note of caution is, don't believe that something is additive free merely because it is labelled as such, or because it is sold in a health food shop. Many products are labelled additive free, but contain citric acid or some other additive. Don't ask me why citric acid gives me a reaction, but it does. Maybe because the additive labelled "citric acid" actually contains other things as well, I am guessing.
Hope this helps your son to get better - happy label reading! Do you have any meaningful support from your doctor?