I have 2 unrelated thoughts.
1) There is absolutely occupational therapy focused on feeding/texture issues. Not to scare you, but for a good friend of mine, these types of early food/texture issues were the first sign of what ended up to be a sensory processing disorder diagnosis. Even if it doesn't end up being that, it doesn't hurt to get a professional opinion on this. You could call your local Children's hospital. They should be able to tell you who in their system works with young kids on feeding issues.
2) It's also possible you are rushing this, and he's just not ready yet. One of my kids was slow to eat solid foods. He screamed when someone tried to get him to eat frosting off of his birthday cake at his first birthday (they were convinced if they could get the icing in his mouth, once he tasted it, he'd love it, so they tried to shove a fingerful of it in his mouth. It backfired terribly.) He choked on puffs and cheerios until he was at least 14 months old, maybe a bit longer. Developmentally, he just wasn't ready. We stuck with breastmilk and formula to make sure his nutrition was good until he was about 15-16 months old. Then, one day, he started eating solid food. He was just ready and the issue solved itself.
I'm also inclined to think you might be pushing him, because you talk about him choking on water from an open cup, sippy cup, and a straw already. My slower-to-eat-solids kid for sure couldn't use anything but a sippy cup at that age, and I know I didn't give him water. Like your son, he was a small child, so I definitely didn't give him anything that would fill him up without nutritious calories. Since you are worried about your son's weight, stay away from water and juice, take a few steps back. Give him formula/breastmilk, and you can even give a bottle once or twice a day if that will help him stay healthy.