My doctor highly recommends it (it's still on my todo list). I have severe allergies and the most important thing is a HEPA air filter. You should get one for the main living area and his bedroom. We find that if you turn it on low at night time (and up during the day) that it makes a really nice white noise machine. My doctor says that my two sons have the clearest lungs she's ever heard in the DFW area. We also have no curtains in the house nor carpet. Wall to wall carpet is actually illegal in some countries because it is an absolute magnet for allergens and toxins.
If it's not realistic for you, then you should at least not have them in his bedroom. And, everything in his bedroom should be washable (i.e., stuffed animals, etc.) Also, make sure that you are using all non-toxic cleaning products Many folks with severe allergies also have chemical sensitivities - air fresheners, bounce dryer sheets, etc. are some of the worst offenders. And, given that you are cleaning the carpets, you should make sure that the cleaner is non-toxic or you could actually be çompounding the problem. Unfortunately, unless you made sure that new mattress was made *without* flame retardent, it is off gassing some pretty toxic stuff:
http://www.mindfully.org/Plastic/Flame/Children-Flame-Ret...
You need a note or Rx from a doctor to have a mattress made without flame retardant.
All the things I mention are of much higher consideration than having the air ducts cleaned and it's not likely useful to do that until you've tackled all these other issues.
Another very important thing to consider since his allergies seem to have started more recently is the air quality of his school. Many, many schools are moldy (even new buildings due to poor construction since schools are built on the tightest of budgets). And, mold exposure can trigger and mimic allergic reactions. Also, mold exposure can cause systemic fungal infections and depending upon the type of mold, the toxins can severely impact one's health.
While I might seem to be over-reacting, I and my sons are only now healthy after having dealt with *all* of these issues.
Be well,
M. B.