A few suggestions - keep trying lots is the main thing!

Try lying him on the floor on a cushion, and lying down next to him. He has to make a "kissy mouth" for bottle drinking - so keep the bottle further forward in his mouth than you would put a nipple. Don't let him get too grumpy - put 20ml EBM in the bottle and try for 10 minutes and then go back to the boob. Keep trying a couple of times a day for a few weeks and he will hopefully get the idea. Can you get him sucking on your finger and then swap it for the bottle?
At this stage his tongue thrust reflex is pretty strong - so his automatic reaction is "get that plastic thing out of here".
good luck - it took us a couple of months to get to the point where he will take a bottle from someone else and drink 100ml (and he was a bottle fed bub for the first month or so....)