Let me give you some good advice. Seek out a certified allergist.
Allergist deal all the time with asthma because asthma is often
triggered by allergies. I had asthma as a child. I also had hidden food
allergies. I was allergic to corn (corn starch, corn syrup, etc.) and
dairy products. I had chronic fatigue too. My doctors did not know
where the chronic fatigue came from. Decades later I developed joint
pain and gastrointestinal problems. I found out when I was 40 that I
had hidden food allergies (by accident). My health problems got better
when I changed my diet.

Food allergies can cause mild or severe reactions. They can also cause
delayed reactions. I'm talking a 72 hour delay. Many people with food
allergies don't know it. Food sensitivities can trigger acne, eczema,
asthma, migraines, psoriasis, sleep disorders, and many other problems.
Most doctors are not trained very well in recognizing food allergies
that are NOT life threatening. Many of the test for food allergies are
not accurate. Their is a debate among doctors about the difference
between "food allergies" and "food intolerence". I don't have the time
to explain everything, but seeing an allergist may be the best thing
for your child. Check your public library for books about food
allergies. It may surprise you. Most doctors will give you the quick
fix (drugs) which may not be the best fix.

Brad_Chad62@yahoo.com

It doesn't appear that any of us have an answer for you, Rebecca.  You might want to contact your sons doctor to ask the question.  Sorry we couldn't be of help, but keep checking back anyway because it's possible someone will still post an informative reply

Dear Rebecca, you would understand that your child most likely has some
form of allergen sensitivity problem and that problem would most likely
involve some food allergy element at his particular young age.
Quite obviously the problem has developed over time and would have been
evident previously in some lesser form, probably sniffling or repeated
cold type events.
I have noted that this kind of thing commonly involves cattle products
in one form or another including dairy items etc. (especially including
cowsmilk based baby formula having been used)
Another interesting finding is that in these kinds of cases a
substantial antibiotic useage history is common from very early age and
the child is usually short term breastfed.
Of course some environmental issue, fumes or polluted air may also have
involvement.
I have seen a similar situation where the mother was wearing strong
antiperspirant and strong perfumes and as the child became more ill the
problem worsened and he required more close nursing which made things
progressively worse, another case involved a "teddy bear" always being
placed with the child whilst he was sleeping.
When these kind of sensitivities are generated they can be difficult to
defuse because other substances also begin to have effect.
I would be inclined to get the child to a completely different kind of
environment and a different food source type as an emergent kind of
action if possible, to see if any condition change might happen. Keep
notes of foods and dates with contents and sources.
I have noted recently that some older children appear to have problems
with fried items which may possibly involve a vegetable cooking oil
type, but so far this appears mainly only as severe stomach pains.
Allergy testing will prove very enlightening for you, but the childs
drug regimen may blur results.
You will have to "get smart" and be extremely observant. Initially
suspect everything.
I would agree essentially with Brad_chad's posted reply.
Cheers, Merlin.