My Son Is 8 Years Old And He Is Weigh

My son is 8 years old and he is weigh 48 lb, He don't have any weight gain since last year. His birth weight was5#2lb. He is tall but very skinny. He had h/o eczema and still he has some. Above all he had abscess in his perianal area on 8 month back, and that wound is not completely healed. Doctor cleared he don't have any chronic GI diseases. He is average eater but very skinny day by day. what may be possible cause?