One of my sons is a very slow eater too. We don't worry about it much, and try to allow for plenty of time and substantial snacks (for when he can't eat much). He needs to take frequent breaks during the meal (he bounces a yoga ball in the adjoining room for a couple minutes and then comes back). Being forced to just sit and eat is sheer torture for him. We do try to teach table manners, but we don't really push them, just information and gentle reminders. When we are at a restaurant/have guests we talk to him beforehand about our expectations and make allowances for his needs (such as a short walk between courses), and he can do quite well when he needs to. But it would take way to much energy for him to be on that best behaviour at every meal, so most of the time we let him eat how he needs to without worrying too much about it.
As far as the choice thing, could you maybe give him just two options to choose from? Or maybe he could just choose the fruit or vegetable? Maybe some small choice like, "Will you have apples or celery?" rather than "What do you want to eat?" wouldn't overwhelm him, and would help him to enjoy his meal more?
Something that has really helped us recently has been to get our sons involved in the food preparation. Once a week, we have a special meal. I encourage them to each choose one recipe from my cookbooks and then I work one on one with each of them to prepare their dish. I really try to stand back and let them do as much as they can safely. They enjoy the sharp knives and the hot stove! I think it appeals to their sense of adventure.
If they start to get overwhelmed from a sensory point of view (for example touching something that is the "wrong" texture), I step in and they take a break. They have AMAZED me at how much they were willing to do and try, and it has really improved their relationship with food in general (it's not quite as scary and mysterious when you see where it comes from and how it's made). Your son, being only 6 may be a bit young for this yet (mine are 9 years old), but maybe not, it would depend on the particular kid.