How little formula is a baby ok to take when unwell?

My 13 week old son has bronchitis. For the last 10 days he's been taking reduced feeds. We are struggling to get him to take more than 2 or 3oz at a feed where as before he was taking 5 or 6oz. Over 24 hours I'm only getting about 17-20oz into him. He's still having wet and dirty nappies. He's 14lbs 7oz so I am aware he should be taking atleast 29oz a day. He was checked by GP yesterday and he didn't seem too concerned he said as long as he has 3 wet nappies a day which he has been.