I had this weird red patch on my chest a while back. It didn't itch or anything, so I didn't think it was eczema. (I've had eczema before and it usually does itch for me!) Went to the doctor and he told me it WAS, and gave me some steroid cream, which I used. It went away quickly then, so he must have been right.
According to
this site, lots of things, including just a common cold or flu, can cause a flare in eczema, so maybe that's what it is?
Hope your boy is okay now!