First of all, it's great that your husband is looking at nutrition and not just medicine - so many doctors are starting to realize that they had little or no background in this while in med school and they are so dependent on medications. He probably knows, then, that the AMA recommended in 2002 that everyone (man, woman, child) supplement because our diets (and our food supply) just ain't what they used to be! And the Physicians Desk Reference will tell him (and every other doctor) that pills (including gummies, prescriptions, vitamins, etc.) just aren't highly absorbable, which is why many doctors call them "expensive urine" (high degree of elimination).
There's been a ton of work in nutritional epigenetics that has helped us all do better with supplementation. Comprehensive supplements (vs. a handful of vitamins that barely meet the RDIs set in the 1940s) and the new superfood shown to affect gene expression have also helped tremendously with kids on the spectrum. There are a lot of studies that indicate that our poor food supply is directly related to the increase in these disorders (ADD/ADHD, Asperger's, PDD-NOS, you name it). In my work, we've seen a lot of improvement in kids with sensory issues in just a few weeks. I just came from a seminar on gene expression and there are well over 70 scientific papers on this alone - and everyone can do it if they connect with the right sources. I spend a lot of time doing consumer education on this. If this is done right, a lot of nutritional needs can be met, so there is not so much pressure on the highly textured foods you're having trouble getting into your child. And the foods we've been feeding ourselves and our kids, even after 30 years of "eat healthy and exercise" are not helping things - the problems are getting much worse!
That said, you can make subtle changes to increase the nutritional value of the foods he will eat. Remember that it takes something like 12-15 exposures to a new food for a child to embrace it, so don't give up the first time it doesn't go over big. Also, HIDE foods easily in other foods! Jessica Seinfeld's cookbooks give some great ideas, and there are others. I was a master at putting things in pancakes, pasta sauce, etc. Make your own nuggets with chicken you cut up yourself, then dip in egg and then a mixture of wheat germ and whole grain breadcrumbs, with or without cheese. Quick fry in olive oil to crisp up the sides (start with light until your son graduates to extra virgin), then finish in the oven, baked on a rack so the bottom doesn't get soggy. You can do the same with chicken patties (use ground chicken and make your own coatings with panko or regular crumbs, or wheat germ, and with fish filets.
Smoothies and gazpacho are great ways to hide veggies. Try kale chips - they're easy, cheap and fun. Try taco bars with healthy choices - kids who can choose their own add-ons often eat them. Make burgers with a lot of added refried or other mashed beans, defrosted frozen spinach (my son thought it was parsley), and some other flavors - if you use really lean meats (I mix grass-fed beef or bison with ground turkey) then you need to add moisture so they aren't dried out from the lack of fat. I use grated onions with their juice, frozen vegetables or leftover watery veggies like grated zucchini, tomato sauce or soy sauce, etc. You can also add flavors to anything that absorbs all the water - couscous, rice, quinoa. Try chicken or vegetable stock, tomato sauce, etc. If he'll eat tortillas or wraps, try adding shredded carrots and cucumbers, spreading hummus or refried beans, spinach, maybe sprouts.
I made falafel for my picky eater - he thought they were nuggets. You can use ground chickpeas, or buy the mix. I didn't like the heavy salt in the mix, so I diluted it with wheat germ, additional chickpeas (ground up) or chickpea flour, etc. Instead of just water, I used the leftover cooking water from steamed spinach and broccoli - nice 'green' veggie water for added flavor and nutrition. I fried them up like the nuggets - quick fry in healthy oil, finished in the oven. I made them with a meatball scoop or small ice cream scoop, but if you can find metal cookie cutters, you can put those right in the oil and put the mix inside them, turning with metal tongs. (Takes time but it's a fun activity for kids.)
Try vegetable and fruit breads - cranberry, zucchini, banana. Try fruit tarts or things like apple crisp with just oatmeal, fruit & a little sugar.
And kids like to dip things in other things - so offer vegetables (raw or blanched) with a little ranch dressing, tahini or hummus. Get a fancy cutter from the gourmet food store, and cut things in different shapes - or cut on the bias. Offer him choices - carrot spears, or carrot "coins" or diagonal carrots, or zig-zag edge carrot. That helps kids realize that the same food can look different and still be the delicious things they like.
You can hide a lot of things in soups and pasta sauces, which you can use on the pizzas he already likes. Make a "salad bar" array of things and then have everyone make their own pizza - try making a face or a design, which will require him to use different vegetables. Start with what he likes - like edamame "eyes" but try to add a red pepper strip "smile" and a broccoli floret "nose". Or make a "garden" with green bean "grass" and cauliflower "flowers" and mozzarella clouds above.
Once you find something he likes, don't change it for 3 more tries. Otherwise he'll get suspicious of everything you do.