I'm afraid I didn't have time to read all the thread but I'll tell you what worked for us, although DS (now 2) didn't have it as bad your poor DS and has now almost grown out of it.
A big no to aequeous cream, it was used a lot in the past and most GPs are not aware it's actually not suitable. For a lot of eczema sufferers, it makes things worse.
You have to moisturise a lot, and findind the right moisturiser is unfortunately trial and error. Weleda is very good for DS, but might not be good for your DS. One trick is to apply the moisturiser on damp skin, so maybe right after his bath. Don't rub it in, just let it soak in.
Oats in an old sock in the bath water worked for DS. Bath products were a big no for us, and the oats helped softened the water and seemed to soothe the itching.
Definitely see a dermatologist. Ours told us that cortisone creams actually sting, so it's better to use cortisone ointments. Sometimes you have to use them for a few days or so until the flare-up is under control, and then moisturising is more efficient.
Finally, we made the mistake of using bio detergent for our clothes (and a strong one at that) and non-bio only for DS' clothes, until we realised that we used to hold him a lot and the cheek he was sleeping on in our arms rubbed against our clothes. The flare-ups were worse on that side, so we changed the detergent and could see the difference straight away.
Try giving up the conditioner at least for a while, as it's full of chemicals which stay in the clothes. Also, a liquid detergent works better for us, as it's easier to rinse off, and we always double rinse everything.
Good luck, and sorry if I repeated what has already been said upthread!