My three children all had asthma of different degrees from severe to mild.  The child who had severe asthma had to go on a very restricted diet, excluding dairy products, additives, salt water fish and wierd things like red lentils from 2 years old.  It was tough at times on both of us, but worth it.  Now he is an adult he can eat most things.  The other two had mild allergies to some foods but not enough to restrict their diets.  So I have to say there is no easy answer, each child is different, keep a diary of what he eats and when he is unwell and that may help you pinpoint anything as sometimes the effects are not immediate.  There were usually other tell tale signs like behaviour change, eczema flair ups etc after food which bothered him, which is why a diary is so helpful.

Good luck and I hope your son is more like my other two children and has infrequent attacks which are easily controlled.

 

http://members.aol.com/katieparks/asthma.htm try this website I hope it helps

 Lexi xxx

All i can tell you is with my now 4yr old and 2yr old i did'nt change a thing i kept a very close eye on them but both of them grew out of it they have not had their ventolin in over 18mths, sorry its really trial and learn from it.

try to avoid preservatives. this site is a handy reference:
http://www.fedupwithfoodadditives.info/features/asthma/asthma1.htm

My husband is an asthmatic and ice cream sets him off but he dosen't stop eating it .I don't buy it very often but sometimes he asks me to get some ice cream in the shopping well anyway that is just something that sets him off ,i have also heard that weet bix can too but my kids love their weet bix especially my son with asthma but he only usually gets his asthma  when he has  a little cold or change of weather.