Is he growing well? if he is you probably don't need to worry (easier said than done I know!) My daughter was similar in that she would got 12 hours+ with nothing, drink 5 oz then puke up until her next meal, (she had reflux & cows milk alergy) I was convinced she wasn't getting enough, but she was growing (albeit slowly) and happy and alert. She's now caught up size wise with her peers.
If calcium is your concern, then as Red says there are plenty of other sources and you could try adding milk/formula to things you cook, mashed potato with milk, pancakes etc
Have you spoken to your health visitor, maybe they could organise for you to see a dietician?
Once my daughter was fully weaned she would only (and still does at the age of 23 months) take about 10oz of formula a day but often 3oz of that goes down the sink.