Also sorry in advance for the ramble. I am very tired!
So my own answers to your questions....
Why???? The balance of the microorganisms in the gut is essential for good health. A babies gut must be colonized with these organisms. Scientists are still learning about how this happens but we do know that some colonization happen in utero and then the rest happens as babies has its first feeds. So the mums balance of gut organisms is important and then what happens to baby after its born is also important. Bub picks up the good microorganisms from your body, your birth canal, your skin, breastmilk and the environment. Things that can interfere with the colonization of baby's gut could be poor health in mothers gut, antibiotics to mother in pregnancy or labour, antibiotics to baby after birth, baby being given anything other then breastmilk and introducing unsuitable food or food too early. Look up the newborn sterile gut. So just to be clear I am not saying that a mum is wrong to have antibiotics, a c section and to formula feed. These things are often necessary or out of our control. Also some babies have all these things and their guts find a way to manage. But for some bubs a combination of these things results in a gut that is not able to get the balance of microorganisms right for easy digestion of food.
Oats are not bad. All grains are a bit difficult to digest. They can also throw the balance further out in a very sensitive gut. Of all grains oats are on the comparably gentler side of things do it doesn't surprise me that this is one thing that your little one can manage. As his diet is so limited I would not cut the oats out right now but you should immediately start soaking them overnight (don't forget to add the apple cider vinegar. It's an important step). But cutting all grains may be part of a gut healing diet. Have you looked at the gaps diet?
I don't think his limited diet has caused the gut problems. How, what and when you introduced solids may have played a part but for some reason he was probably out of balance already. His gut sensitivity is definitely part of why he doesn't want to eat much variety of food. He knows that different foods cause discomfort.
Just out of interest does bub have skin issues? Rashes or eczema?
I'll try sending you a PM about someone I know in WA.