medical help!

So to cut a long story short. My son has been at our local hospital for a week due to loosing 200g in 4 days. He refuses to eat more than 50ml per feed. They had us feeding then having a 15m break then feeding again. It worked for a few days but he kept dropping weight. They tried extra calories which made him put on 150g but he didn't like the taste so they started adding extra scoops of formula to his bottles. He dropped 5g. They sent us home yesterday. Even though his poo is black and he is very hard to settle. I asked for a second opinion and more tests to rule out malabsorption and got told it wasn't needed. I'm really upset with our care and want to have him seen by a specialist at the RCH. Does anyone know how I go about getting him seen? Do I just take him there or do I have to get a referal? He only weighs 6.3kg at 5 1/2 months. I'm concerned he will gets even sicker if we ignore it!