Might be delivery method. Are you trying bottles? Try a sippy cup or straw cup. Give it to him at meal time in his high chair, not laying down cradled. My first son started to be weened to a sippy at 6 months and we were done by 9. He usually didn't drink more than 2oz at a time. He was a great eater and snacked on his cups all day so it seemed fine. Formula consuption was at the low end of ok but he was in no way lacking in nutrition. Second son got to like formula from a bottle buy sitting in a bouncy or in the baby bjorn while I chased the older brother around. Second boy the formula in the cereal thing (see below) helped a lot.
Are you doing baby cereal? Use the formula to mix the cereal (how about a nice banana, avocado, baby cereal formula mix). Then he will get a better taste for it. I found the best luck I had with both my boys was to not associate it with breast feeding. It is something different. A totally new food. Delivery needs to be new too.
If you do do bottles, I highly recommend the playtex nursers or any other bottle with a bag. Sitting up or waking around with the bottle is so much easier. You don't need to tip it back at all.