Severe stomach pain, cramping, burning and vomiting

My partner is 26 (27 in April) and is suffering with severe stomach pain, cramping, burning and vomiting. it comes in episodes and can last anywhere from a couple of hours to a week. It's been like this for the past 6-8 months and finds it hard to put weight on. He's had blood tests, a scan (similar to those pregnant women have) and tests for Hep C, HIV etc and everything came back negative or that there was nothing wrong. Antacids simply are NOT working and he's in severe pain as I type this. Please, can anyone help??!!?Drinking water does not relieve the pain and nor does milk.