should i breastfeeding for my baby after he on formula for 3months?

Hi mummies. When my baby is newborn i tried to pump out my breast milk for him to drink but after 30mins he vomitted all out.(as per nanny told me) I gave him tried 3 times but still the same. I dont know is he not suitable or what. I feel bad... i dont know is my prob or the food that i ate that makes him vomit. I know breastfeeding is very good for babies. Recently he fall sick. He just 2mths n he sick. Can see that he is not strong as breastfeeding baby. So I'm thinking to breastfeeding back him or just continue on formula as he alr drink for almost 3mths. Kindly advise. I dont know i still can pump out breastmilk anot?