I've done a lot of work with my ds (AS, age 9) over the past few years and he has a good nutritional diet now. It's still a bit rubbish with social foods though, mainly because he doesn't do food combinations like pizza, burgers etc. I've only just got him to accept sandwiches and breakfast cereal in the last four months!
It's useful to work on obsessive interests as motivation, use lots of praise and rewards, my ds responds well to charts and visuals of what he's working on. We do one or two foods at a time, gradually introducing them by discussing each one first, then I involve him in coming shopping to buy them. We draw up progress monitoring charts together, he gets to edit them if necessary.
As for trying the foods themselves, we start with just tolerating them on the plate, building up slowly to touching them, licking, holding in the mouth and eventually swallowing. It helps to give a lot of notice about when you'll try the food again, again with a visual timetable if possible.
I can help out with some advice and charts etc but probably not for a couple of weeks. I have a lot of problems to sort out for my elderly mother atm so I'm not very available.
Email me at [email protected] and I'll keep you in my inbox, then send you some stuff when things quieten down.