My daughter will eat mince meat, cut up chicken or beef sausages, cut up or diced lamb or veal, plus she loves pumpkin diced, brocolli pieces, potato diced, green beans cut up about 2cm long, and if I make a cheese sauce, she loves to dip her vegies. I have also found my daughter will eat fish fingers if I put some tomatoe sauce on her tray so she can dip them... she loves her "dip-dip sauce"
Another thing I tried recently was to cook some pasta... the vegie spirals one (different colours in one pack) and mix with grated cheese. She loved it... not sure what you can use though if you are dairy and wheat restricted... this makes it a bit tougher for you. I think you can get special pastas at health food stores... no wheat?
Will your son eat fruit? I find if I dice up fresh apple, pear, bananna, watermelon... my daughter will give them all a go. She especially loves green grapes.
Have you tried going to a health food store to find alternatives? They might be able to help you there.
What about baked beans and things like that? Baked Beans - salt reduced are a good source of fibre.
I would just keep putting different things in front of your son and let him show you what he likes. But don't offer a different meal at the time he has rejected the meal in front of him.
Goodluck... will let you know if I can come up with anything else.