If you tried expressing and he won't have that, it may be the bottle he dislikes. I'm assuming he drinks breastmilk without any fuss, just won't take it from a bottle?
You could try other bottles, etc. But honestly, at 6 months, you might look at sippys or straw cups or even open cups. If you can, try and give him EBM for a while out of a new cup along with a formula so you can see if it is the cup or the formula, you know?
Do you have to give formula if you are able to express?
Oh and I've never FF, but I believe babies can go through several before they find one they like.