My son 13 months old. he has red color eczema started when he was 4 months. it was spread all the body but now on back of(knees, elbows,nick)?

Not unusual: The distribution you describe is typical for eczema in his age group. Continuous use of a moisturizer of the affected areas is critical, as well as a burst of a topical steroid (hydrocortisone) for flare ups. Also maintaining at least 40% humidity in the house. If these do not improve his situation, you should consult his pediatrician.