My son has a rash mainly on hands both sides feet around ankles, front side of knees few up arms and face. blister like doc doesn't know what it is?

Hard to say: The distribution of the rash and description includes many possibilities from an allergic contact dermatitis to eczema. Infectious processes like enteroviruses can also produce blister like rashes with odd distributions. Keeping the areas involved clean so skin bacteria don't cause a secondary infection is the primary management. If not improving soon get a second opinion.