Do you know what he struggles with, is it the taste or texture? Can he cope with sauces? If you know what the issues are for him, it might be easier to suggest what might work eg if the issue is texture, if he will eat cauliflower but not broccoli, you might find serving him the broccoli stalks without the flowers helps. What veg does he tolerate?
Does he eat fruit? Upping his fruit intake will help with fibre and vitamins etc.
Will he eat rice? You could try adding cauli rice to normal rice and make things with sauces that have hidden veg eg spag bol but with rice for your DS instead of pasta.
You can add veg to pizza sauce under the cheese. Cheesey mash with things like parsnip, carrot, swede might work as the cheese masks the taste and the mash masks the texture.
If he likes baked beans, you could try making homemade baked bean sauce to serve with veg (try to make the pieces soft and baked bean sized).
TBH, don't stress too much. If you can find a few things that he will eat, just serve all of them every day eg a carrot in a smoothie, baked beans on toast for lunch or breakfast, cauliflower with every dinner, a couple of portions of fruit (preferably different colours). A small range of veg every day is better than trying new things and him skipping the veg totally.