Lea, yes, there is a connection between antibiotics and candida. If the problem's candida it's pretty obvious - lots of farting! with all that yeast bubbling away inside you. If in doubt, try not eating any sugar or yeast for a few days, then eating a lot at once. You won't necessarily feel better, because there hasn't been time to get the yeast out of your system, but while you're not eating sugar, the yeast goes quiet, and when you eat the sugar, then it'll all explode into life again! This condition can cause an amazing variety of symptoms, but there are some quite good books on the market which list all of them - get one at a health food shop maybe. One symptom which is quite unmistakeable, but a child might find difficult to describe, is the unreal feeling, for example, "I look at my hands typing this, and it feels as though I am looking at someone else's hands. When I walk around I feel as though I am in a glass capsule and can't connect to other people." If it's candida, the good news is that it's quite easy to clear up, though may take some time (several years). Basically, you have to starve the yeast by not eating sugar, and get your body to recover from the reaction to yeast, so no yeast either, and maybe homeopathic remedies to strengthen your immune system til its better. Eventually, the balance in the gut gets back to normal, ie small amounts of yeast, but not out of control, and you can start eating sugar again. One associated problem is that if you get really down due to reacting to all that yeast, you may then get other food reactions, which have to be treated as well. But you can definitely make a 100% recovery from this condition.
If you're going to explore the food additives/candida aspects, I think the best thing you can do is get a few books, and for additives, visit www.hacsg.org.uk for more info. You can then make a more informed judgement about whether either of these might be at the root of your son's problems. I think all the other advice given on this board is excellent too for treating the symptoms, the two approaches go hand in hand.
Good luck with this - please do let us know if you find out what it is! or if there is any more info that I can provide, please do ask.