This guy sounds very ill.
Be careful and think any advice anyone may give you through very thoroughly.
I'll state the obvious here, and you already know it.
He needs way more water.
He needs way more fruit.
But honestly here, this guy sounds so sick that he needs professional help.
Find a natural hygienist, or send him to the Hippocrates health institute.
He can't take care of himself, and it sounds like no one else can do the job either. He needs real help.
Also, and this is pure opinion here and by no means intended to be insulting.. It sounds to me like he has a lot of mental issues going on. If he is okay with how he is, then there is something wrong.
If he 'get's irritated' when you try and feed him.. this is wrong. There's something backwards in his way of thinking. If he's not begging for help and taking all that he can get, there's something wrong.
His negative health may have caused the problems of thought, or the mental problems may have caused the poor state of health.. but it's really the chicken or the egg here, it just doesn't matter.
Now, if you don't go to a professional...
I'll give my advice, just because I like to talk.
I like to post on forums and look the the guy who knows everything. It's a past-time. I like to help people, too, but really it's all about me. That's why I'm here. Anyways.
If he gets full on 1 banana, or 1/2 a mango, then I argue that the problem isn't with him, it's with the fruit.
Here's how the body works when deciding what to eat.
If the energy your body has to spend to digest is more than what you will get out of the food.. it will be unappealing. If your body doesn't have enough energy to digest, it will be unappealing. On the flipside, if the food is super easy to digest and provides great energy, it will be very appealing.
This guy sounds so sick and week, that he needs easy-to-digest energy.
Laugh all you want, but the most easily absorbed energies are: Love and support, Reiki (hands on), and the sun (infrared).
These things with enough clean water and ripe organic fruits will work wonders for anyone, especially someone in his condition.
(clean water means check your source of bottled water. not all 'spring' water is actually from a spring. look into your brand) (and ripe organic fruits really means ripe. You can't find ripe at the grocery store. You need to find them locally. Preferably biodynamicly grown)
This is serious stuff here. And it's going to cost money.
I had more to say, but I gotta run.
Honestly from my humble point of view and limited experience it sounds much more like he's just simply undercarbed and dehydrated, and lacking in sleep sunshine and subsequently, exercise. Irritability is the first sign of being low on carbs. Yes I know he has a condition and that alone can make you cranky for sure, but again, both things are stemming from the same problem in my experience. I've seen it first hand, someone who refuses help, is extremely irritable and negative, even overtly hostile, but when they eat some f'ing carbs they cheer the f up. I would say as a first stage if he had to just eat sugary low fat foods, whatever it is, heck even gatorade if necessary to "crank the engine" up to idle at least. GMO oils and GMO potatoes cooked together at high temps with MSG and salt is not a good look for someone with a digestive issue. I mean look at the studies of what GMO corn did to pig stomachs and rats. Cancer, inflammation and chronic disease. If he can't go with the fruit, go super hydration + McDougall style menu, including lots of fresh raw salads as well. No nuts.
Get a copy of the China Study to the parents, and explain that the WHO did an extensive research into protein requirements and found that 2.5% of daily calories from protein is all we need, then they doubled that recommendation to be on the safe side, to 5%. A banana has about that much, as do rice and potatoes. Ask them if they can show you a single clinical diagnosis for actual protein deficiency. Ask them where do gorillas get their protein, or rhinos or elephants. The myth that you can't get all essential amino acids for humans from plants has been proven wrong. Read the China Study and also 80/10/10. They are very well written interesting and easy books to read.
The best way to get used to eating enough calories from whole fruits is to eat them very ripe and/or blend them.
When I first went vegan I lost maybe 5 lbs but now I've managed to gain 20 lbs of muscle to reach a healthier 140 lbs.
I'm only 3" taller than him and I eat 4-5000 cals every single day.
If you eat 700 cals/day you will DIE it's just a matter of time.
First you have to make him understand this, if he wants to live at all..
My sister's 14 month old baby eats 1000+ cals/day for dog's sake..
The problem is in his head not his stomach obviously so I'd start by finding a good therapist.
Well I'd vomit too from all that junk phood.. he won't vomit from high carb, low fat vegan whole foods that's for sure even if it's not all raw.
In my experience Barbara Brennan certified practitioners are the *best* and most effective. Not to say other methods wouldn't be effective, but definitely be very careful and critical of the woo woo crowd. I had incredibly positive results and so have others who tried it. If you go to barbarabrennan.com there's a list there of practitioners in your area. They combine energy work with psychotherapy and do a four year course to get certified. In my experience what they do is not woo woo :)