Nicky, I hope your GP has provided you with a list of food you need to avoid if your ds comes from a family with a history of allergies.
The ones that I remember at the moment are:
-No egg before 1 yr old
-No glutten before six months
-No cow's milk befre 1 yr old
-No whole nuts until 5 yrs
-No orange juice until 1 yr old (not sure about this one, but definitively not an early weaning food)
-And if you can, avoid the Dirty Dozen (look for it at the Organix web site)
DS is allergic to tomatoes, nuts, egg, and has a loooong list of food intolerances. THe reactions vary considerable for example, DS can eat tomato as long as it doesn't touch the skin. But it's always good to err on the side of caution and remove the ingredient all together.
Something that worked really well for us was Gina Ford's The Contented little Baby Book of Weaning as it has a very good program to introduce first foods that allows you to try the ingredient for three days and check for reactions. If you keep a diary you will be more likely to spot which foods do present problems. That doesn't meant that he will be allergic to that food all his life, you may try them again after some time when the gut may be a bit more developed, though in some cases you will be asked to remove the ingredient completely until your DS has been diagnosed.
I wish I could tell you that access to allergicians is great but it's quite the opposite, until you get an appointment (that could take years in certain parts of England) your best friend is the food diary.
Hope I have been of help.