Resistance to breast milk is practically unheard of. Breast fed babies may pass 8-9 stools normally per day. Despite that they gain weight well and thrive. This does not mean diarrhoea; and breast feeding should not have been stopped in case of your child. Top milk, on the other hand, carries a risk of diarrhoea if proper hygiene is not maintained while preparing or administering the feed. Also, if formula feed is not diluted as per instructions or inadequate amount is given, baby's weight gain can falter, as seems to have happened in your case. You should start breast feeding again without hesitation. Hopefully milk flow would be adequate to meet the babies needs. Otherwise, you could supplement him with undiluted pasteurised toned milk with sugar added. Try to give him using a katori/ cup rather than a bottle, which again carries a risk of diarrhoea.