How to deal with my boyfriend's son moving in?

I've been dating my divorced boyfriend for three years now. About 3 weeks ago he finally proposed. I was starting to plan the wedding and all of the sudden his 16 year old son is sent to live with us for and indefinite period of time. This is very bad because his mother just decided that without consulting with my boyfriend. I questioned my boyfriend about this and it all went to a very bad argument. I'm stuck in a rut and don't know if I should just leave.