No, don't worry, I haven't taken away his formula!
My 8 month old just got his first tooth, and while it was breaking through he completely refused his bottles. So I offered him his formula in a straw cup instead, and with a bit of encouragement (and lots of small feeds a day) he managed to still drink enough formula for his needs.
Now his mouth isn't bothering him any more, but it seems like he decided he prefers the straw cups over bottles. I think because he can sit up straight in his high chair and look around better while drinking. Personally I am very much in favour of always holding baby and bottle while feeding, but it looks like he's done with getting his formula that way.
I think I'll just let him have his straw cups instead of bottles if that's what he prefers, but is there any reason I haven't thought of why I shouldn't? He seems awfully young to be off bottles. I do already make sure I hold and cuddle him extra to make up for the lack of physical contact while feeding, and I don't just hand him the cup; I sit with him, holding the cup for him.