T., I am so sorry you guys are having to suffer through this. I really feel for you!
I know you said, "we have also tried changing his diet, limiting things with sugar, dyes etc., to no avail" and "they told me he has asthma" -- both of these things throw up a big red flag and scream undiagnosed ALLERGY.
When you tried to change his diet, did you put him on a total elimination diet (TED)? Or did you limit things, but not remove them completely, from his diet? I ask because it would make a BIG difference: limiting certain foods (just serving smaller amounts) would still mean that your son could react to those foods (and no behavior change), whereas totally eliminating certain foods from his diet would mean your son would have a chance to NOT react (thus a change in his behavior would be noticeable).
I urge you to read, if you haven't already (and if so, I apologize for the redundant suggestion), "Is This Your Child?" by Dr. Doris Rapp, MD. A total elimination diet would be the fastest way to get your son to a baseline (where he's not reacting to something).
If a total elimination diet seems too overwhelming, the big allergens that I would remove to start with (if I were in your situation) would be: 1) corn - and that means *everything* with corn, high fructose corn syrup, corn starch, etc., 2) wheat (also known as a GF or "gluten-free" diet), and 3) cow's milk (including casein, whey, etc.).
My son has a dairy allergy (he's 10 months old), so we've had to go through the whole eliminating dairy bit (anything with butter, cheese, yogurt, sour cream, milk, cream, buttermilk, milk powder, whey, casein, and any and all derivatives!). It's really 2nd nature to us now, though at first it seemed daunting. To help me through that, I started talking a lot with other moms who have been through food allergy issues before since it was relatively new to us. Based on what these moms have shared, if I were in your place, I'd eliminate corn first because more than a few moms say that corn, especially corn syrup, results in their children really just losing control, becoming angry/voilent, not able to control their impulses at all. They say that without corn, their kids are pleasant, wonderful, calm little people. But with corn, it's like they're crazed, angry. So... for what that's worth. It may not be the issue for you - but if your son has been eating corn in all of it's different forms (and corn, cornstarch, high fructose corn syrup are EVERYWHERE, even in things you wouldn't think of), it might be worth investigating further.
Some resources (again, I apologize if you've already seen this stuff) for allergy & corn, dairy, and gluten free eating:
http://www.vishniac.com/ephraim/corn-you.html
http://www.cornallergens.com/new/basic-corn-allergy-steps...
http://www.godairyfree.org/
http://health.groups.yahoo.com/group/GFCFKids/
Best of luck to you - I truly hope that you and your son are able to find some relief, whether that's in the form of medication or major diet/lifestyle changes!