There really aren't any set guidelines for when to offer solids first. Some do it that way from the very start, some start between 9-12 mo, and some keep doing breast/bottle first until whole milk is introduced at 1 yr. It's really up to what works for you and your DS, but formula should still make up the main part of his diet until you start whole milk.
You can switch entirely to the sippy if his formula intake stays where it should (around 24-26 oz). If not, I'd stick with the bottle until solids are the main source of his nutrition, which usually happens around 1 yr.