I would think if it's food causing it, it would be something that you are offering only at that time of the month that he has problems. During the weeks he had no diarrhoea (initially before you started changing his diet) had you dropped something from the diet and then put back in for the week he had problems? What else happened at 5mths of age - any medication, vaccinations, new food? So you have now removed the offending foods that initially set it off and the oats are the only problem? Gluten is found in oats, wheat, barley and rye - oats aren't really a true trigger - it's controversial whether it's a problem or not, how does he go on bread and pasta? I would think they would be bigger problems if it were gluten.
But in reality, any aspect of a food can be a problem, gluten is just one protein, so if the problem seems to be with oats then it could be another part of them. I still wonder though because you had 3 good weeks and then 1 one bad one - seems kinda strange, more like there is something that you're randomly giving him instead of on the standard menu.
My dd had did a gluten challenge before having the Coeliac screen done (at 12mths), so I believe it is must be reasonably accurate, but I will rechallenge her every year to see if it develops later as I think there is a problem, but it's not showing up yet. I have read that it's not too accurate until after age 2.
