My son has allergic rhinitus, asthma so should I get rid of our feather sofas?

Dust mite allergy? Certainly consider this is your son is allergic to house dust mites and spends a lot of time on that furniture. But if your son is allergic to house dust mites first cover the bedding - all pillows, mattress & comforter in miteproof encasements & remove carpeting throughout the house. If that doesn't work think leather sofa. And don't neglect allergy immunotherapy.
Maybe. Depends on the severity of his allwergy and whether feathers are in his allergy profile. Talk with uour doctor.
No. Believe it or not, most symptoms like this are caused by confusion in the lower (automatic) brain, most often caused by lousy diet, especially all that sweet junk that seems to be so popular to all our kids. It tests credibility but I have seen it work all too often!