Hi, Rosie. This is a massive amount of formula - honestly. There is no way anyone can continue to breastfeed giving these large quantities with a baby as young as this, and it is a real shame no one helping you has explained this fully and enabled you to keep formula to a minimum when you were told to start giving it.
The good news is your baby is still happy to go to the breast on most occasions.
You will need to work out a plan to rescue your breastfeeding, by gradually reducing the amount of formula you give him, while at the same time hugely increasing the opportunities for him to feed on the breast. You can look up 'relactation' on the web or in the mumsnet archives for ideas, as well.
He needs to come to the breast and to have at least both breasts as often as you can do it. If you keep him in your arms/in a sling you will be able to respond to every little feeding cue with the offer of the breast. This needs to happen at night, as well, so the 'rescue' mission is not an easy thing to do...but it can be done, over a period of a week or two, with pretty constant bf, with formula only when he needs it. Your health visitor/midwife should be part of this, to help you work out quantities to offer. You could start by asking them if it would be ok to offer formula feeds of no more than 60 mls for instance.
You can also phone any of the breastfeeding lines - they will also help you talk through your options.
Hope this helps.