So sorry to hear about your brother! How awful.
As a nurse I have tube fed many many people over the years, and you are absolutely right, that crap that they use is terribly full of rubbish ingredients. If you've ever smelt the liquid, you'll be instantly disgusted, its just awful. I've never seen any brands that contained anything natural or unprocessed.
It wouldn't be too hard to make your own brew though. The problems I can see with it would be a) figuring out the correct amount of calories/nutrients/fluid for your brothers needs and b) preparation/storage. If you do figure out a brew it would likely have multiple ingredients which would take some time to prepare, so you'd need a nurse or dedicated family member to do this regularly, and most nurses would not have the time to do this. Using your own ingredients would also mean that it would likely need to be refrigerated or made fresh.
See if you can get a dietician onside with you, preferably one who has experience with tube feeding and one who will support you when dealing with Dr's and RN's who may not be so supportive.