After our paed gastro appt yesterday it seems poor old DS is suffering from IBS, am very grateful it isn't coeliac disease and ultimately am glad that she doesn't want to biopsy him (although the thought of preparing the same meals as everyone else for 2 months was slightly appealing lol).
She examined him very thoroughly, am very impressed with her actually, wish we'd got to see her ages ago, and her final diagnoses is intolerances to wheat, gluten, dairy and egg (apparently egg can be linked to the others - not heard that before?) with IBS which explains the other symptoms.
I had no idea that children as young as 6 (well younger as DS has been suffering for a long time) could even have IBS.
We are managing it ok and basically are to continue unless IBS gets worse or he develops other symptoms.
At least we finally have an answer!