Weaning from EBM to formula

My son is 6mths old and I have been expressing the entire time. He has a heart condition so he's not allowed to breast feed as it uses up more energy than bottle feeding and initially he was fluid restricted.

I'm hoping to take 5 EBM feeds to 3 EBM and 2 formula feeds over the coming month/s. His condition is extremely time consuming, as it is, so I'm hoping to get some hours back in my day so i can at least get some house work done.

However making the switch is proving almost impossible. The most he's ever taken from a formula bottle is 40mls and he then refuses to take any feeds for some time after.

I'm looking for any tips, or advice on the best way to do this. Are there any formulas that babies like better? Times of he day that are better for weaning? Any help will be greatly appreciated.