If Breast milk is sufficient and baby's weight is gaining you can give only brest milk till 6 months completion. May be he is not tolerating other feeds. If brest mil is less you can try formula milk like similac-1 or lactogen-1. Try different brands. Weaning can be started with cerelac rice. one or two days vomiting can be there at starting. You have to make cerelac very thin initially to tolerate it. slowly once he tolerates it you can make it thick step by step. Hope this helps.