Impetigo can be found anywhere on the body. My friend had it in and around her belly button once!
To be honest Marie I would say that his severe excema has undoubtedly been a cause of the blisters as you say they are not everywhere on his body.
If you can rule out i.e things like if hed been playing football or running around in a field, or in someone elses back garden and his legs have come across a flower/plant that has caused an allergic reaction...
rule out any possibility of something causing an allergic reaction first as his excema is severe and anything could irritate it.
Try and find out from the dermotologist what type of excema he has, as there are many different treatments for different kinds. I reckon a corticosteroid would be more helpful as well as the emolients.
I also reckon he has one of the most common forms of eczema which is 'atopic' dermatitis, or AD, This can occur on just about any body part. AD can be very painful and physically unattractive, causing skin redness, swelling, cracking, weeping, and scaling.
You may find it gets alot better using the corticosteroid.
Use the hydrocortisone on the face, neck, hands and the flurandrenolide or betamethasone dipropionate in lotion form on the torso (his chest, tummy, and back) The dermo will prescribe the last two.
They all will have a limited time that you can use them for,normally 5 days and you should see an improvement within 3 days. The longest he should be on one for is 7-10 days at a time.
Dermotologists always try to use the lowest dosage possible, but you have to try and make them give you a higher dosage to trat the stubborn crusty skin as lower potency topical corticosteroids wont budge it.
Never use them for more that 10 days as there can be side effects, and the rash can come back worse.