What is he eating? Many times extreme itching may be caused by something in their food. Might be worth a shot to put him on a Limited Ingredient Diet, with a novel protein (duck, venison, rabbit, no chicken or turkey or beef) and grain free. It will take a few weeks to see improvement, but if you do see improvement then it is likely a food allergy. Also be sure to rinse rinse rinse when bathing.
A little coconut oil on his food and rubbed into his skin will also help.
I know that my two are sensitive to chicken, most fish, beef, and grains. It took me a while to figure it out but they are good now
I hope your baby is feeling better quickly. If there doesn't seem to be a food issue, then a visit to a vet dermatologist would be good. Good luck!