6 month old son vomiting 5 days now?

It could be lots of different things. It could be that he has acid reflux. It could be that the sudden change in formula has upset his stomach. It could be an ulcer. It could be an illness. It could be something as simple as gas and he's not being burped often enough. There's no way for any of us to know. All I can recommend is speaking with your sons pediatrician and demanding answers. If s/he can't give them to you, then request a second opinion. Obviously, something is going on.