How can I get my baby to take bigger volumes in bottles?

My son is 7 months old. He is 18lbs 9oz so obviously gets enough but I feel his milk intake is poor. I can't get him to take more than 4-5oz at feeds. The formula box says he should now have 3 7oz feeds with food. For example today he had:

6.30am - 4.5oz bottle (wouldn't take anymore although slept all night)

9am - porridge with 2.5oz milk

10.30am - 3oz bottle

13.30pm - potato & cheese & a yogurt

15.30pm - 5oz bottle

17.30pm - few spoonfuls of pureed Spag bol

19.00 - 5oz bottle

Total of 20oz formula

How can I get him to take bigger feeds at once?