(I wasn't sure whether to post this here or on the fitness stack exchange, so if this is in the wrong place, could someone please migrate the question to the appropriate site?)
I'm a male in my early twenties, but only weigh 89 lbs (very underweight according to BMI standards). The most I've ever weighed was probably around 110 lbs back in early high school, so I've always been on the leaner side, but it seems like I've reached a low point.
At this point, I can barely consume over 1000 calories a day. If I eat too quickly or too much, I get very nauseated and feel like throwing up (but never have so far, as I'm slightly emetophobic). I've spaced out my meals, but it often takes me 2-3 hours just to eat a 300-or-so calorie meal.
I've talked to my GI doctor about this and I've done an endoscopy and a stomach emptying test, but neither test found anything significant.
To try to get more calories in, I've begun staying up very late (sleeping around 2-3 AM) to eat a few snacks to try and bolster up my weight. Since I don't have a job at the moment, I can afford to wake up at 10-11 AM, but that results in eating a smaller breakfast. Does sleeping less contribute to weight gain, and if so, should I continue doing this?
Nearly everyone I've talked to about my situation has told me to "exercise more", along the lines of thinking that more exercise -> faster calorie burn -> hungrier -> consumes more food. I've been walking around the house/neighborhood for 15-30 minutes every 2 hours or so (I don't get any exercise otherwise), but is this doing more harm than helping? My doctor just went with a vague answer of "if you feel like it will help" or something like that.
I live my parents who are both 60+ years old and diabetic, so my diet tends to be limited to what they eat for lunch/dinner (which is almost always lean, low-sugar foods). Furthermore, my stomach seems to tolerate fatty foods (deep fried stuff, fast food hamburgers, thick cheese/butter, oil, nuts) less than other kinds, whereas I had no problems with these when I was a kid. Is my aversion to high fat foods something psychological, or could there be something physiological behind it?