I'm sorry to hear about your little guy, so young to have arthritis. I'm not sure how they treat a child his age, but when I'm in a serious flare steroids are about the only thing that will help. I also find heat very helpful in the form of heating pads, hot baths and the hot tub. Cold doesn't work for me and during the winter months, the colder it gets, the more painful my joints become.
I hope someone that also has a child with RA may have some helpful advice for you, sorry I couldn't be of more help.