Formula

Can I try out a different formula without telling pediatrician or will I hurt my baby....reason asking is I been telling pedi that my LO hasn’t been eating well one said it was ok that’s his response for everything that’s why I decided to switch pedis but this new one said it was ok too. He said he was teething no he is not I would know. It was about lactose intolerant and he has those symptoms and I’ve had that feeling he would be lactose intolerant but they don’t seem to care or understand I can a can here should I try? Or will it hurt him??