It's bewildered.com....bewildered?
I understand completely about the tube after all the other traumas.
Cheesy wotsits are a big hit with DGS 
I think it's some progress that he will eat something and is developing that all too important skill of chewing!
I am thinking outside the box here, hence to gastrostomy suggestion. (Hate that box expression 
). Anyway children can taste what's in their tummy usually with a little bit of reflux so taste is important. Formula milk doesn't taste especially nice and has very little taste variety compared to food. Another observation is that if you eat well and get regular full tummies it encourages your appetite. Which is why obese people can actually continue eating when they are stuffed and anorexics have very little appetite. That scientific btw
In other words eating food makes you want to eat food.
We belong to a Facebook group called Blended Diet UK where we do not feed formula because it can aggravate reflux, doesn't encourage real eating and is full of processed crap .... I take that back it is full of crap!
We all blend real food, or use such things as Ella's kitchen pouches, baby rice, stewed apples etc and put it down the tube. There are more and more professionals accepting this gives healthier children of normal weight, without reflux or constipation, but it's a bit 
to some.
We know mums on this group who have used ng tubes despite them being so narrow but not us personally. If you want to join and get some ideas you are more than welcome. It might just be something that works to get DS interested in food.
I actually think that regardless of what you do DS will be eating fairly normally within a year, especially if he goes to nursery and sees other children eating. Overall I am very optimistic for you but understand totally the frustration of having come this far and stumbling a bit. Believe me it is just a stumble 