Oh, I forgot to mention the reinforcers. Aidan has two main "reinforcers" for cooperating with us. One is video games (which he is hopelessly addicted to at the age of 5) and the other is candy. We use candies like skittles and M&Ms so that we can give him one or two small pieces for doing what we ask of him. (That way he isn't getting too sugared up.) We will remove video game time if he makes a bad choice. (Usually that's for wetting his pants while playing, though. Mostly we would give him extra video game time for trying his no thank you bite.)
Sally44 is probably right about an occupational therapist being a great resource. My grandson has classic Autism and we have to fight with him on food issues too. He is *extremely* resistant to vegetables, even corn which most children like. His school works with him on this and we follow their lead. What we do is require a "no thank you" bite of food. We had to build up to it though. First he just had to "kiss" the food that he rejected. Then he had to touch it with his tongue. Now he will take a bite. We watch for textures that he particularly hates and don't push those issues -- like mashed potatos. lol Our entire family has textures we simply won't tolerate -- for Aidan's mom it's "slimy" foods (like guacamole dip) for me, it's cooked mushrooms, so we're sensitive to what Aidan is going to really resiste eating. But a single bite over time can lead to great variability in food eaten. Aidan eats salad, sometimes carrots with ranch dip, and a wide variety of meats. He adores noodles of all kinds and I will make homemade fried chicken tenders for him because he really loves McDonalds chicken nuggets too.
One option for vitamins I would suggest is gummy vitamins. They look, taste and feel like candy so most kids love to take them. Good luck and don't give up. With autism patience and persistence are virtues not to be taken lightly.
The person to get involved would probably be an Occupational Therapist.
If he is having such problems with food I suspect he has sensory issues with either the taste, texture or smell of food. Or he has rigid behaviours and routines that allow him to only eat certain foods and only when they are presented in a certain way.
What are his other senses like? Is he okay with sound or does he cover his ears or appear deaf. Is he touch sensitive or does he appear not to feel pain. Does he have to wear certain types of clothes and have tags removed etc. Is his balance and co-ordination okay?
If he does have sensory issues these can fluctuate between being over and under sensitive throughout the day.
My son has trouble eating mashed potatoes as well, we are in the process of getting him diagnosed because of other problems- all relating very closely to Asperger's disease. We are very creative in our menu options and very against fast food. For one thing we can't afford it!! Try looking up kid menus on the net- there are recipes and meals that have silly names that will attract kids to eat them. Any kid would find more interest in "sea shells and clam sauce" than just mac-n'cheese....ya know? Hope this helps and hope you find something that works! Best of luck!