Childhood Diseases

Hi Doc. My 3 week old son was breast fed and topped up with Similac HA. He ahd runny stools with mucous. His paed suggested that mom change her diet and cut out allergens if she wants to breast feed or cut out the breast milk and use formula. we went with formula. Initailly he did ok on Similac but then stopped drinking and a few of his stolls were green. We are now on goats milk and he is doing great. Nan HA is not halaal so we can' t use that. Is he, at age 3 weeks, too young to go for an allergy test? how do we know if he has an allergy to dairy or if he is intolerant. He also has bouts of colic and battles to pass wind. we have tried telemant drops, gripe water, home remedies but nothing worked. we are now using buscopan for infants. Is the colic and milk intolerance/allergy related?