Would be easier to answer this if we know what he does eat? eg if he likes bolognaise, it's easy to add grated/chopped veggies for spaghetti bol or lasagna. If he likes eggs and toast can you add sweetcorn to scrambled eggs or give him cooked sliced veggies to dip (I hate this - but DH's mum did both for him).
I do fruit skewers for parties, bbq etc and I'm always getting parents telling me that their kids don't usually eat fruit but they love the skewers. You could sweeten the deal with something to dip, but that takes away some of the health benefits.
Pasta sauces, as you've identified, are a good one. It's worth buying pre-chopped frozen veg (or do your own in bulk and stash in freezer) to minimise the hassle when you're preparing them.
I haven't done this yet but have been meaning to make veggie burgers (I have a chickpea recipe which is a bit of a faff but they freeze well so can do in bulk then just fry/grill) and try buying veggie sausages (not quoin). I suspect this would work for DS but not for DD hence haven't bothered yet.
Definitely consider adding veg to juices. If you have a juicer, this is even easier but I hate ours because of the cleaning up. We make smoothies with frozen berries, bananas, plain yoghurt and watered down with water so it's a less less fruit/sugar intense and less hassle and the kids love it.
Fortfiied cereal might help with the nutritional element, if not the "getting him used to it" element.Not sure how much they're fortified but I've heard people talking about that as a benefit.
Apples with peanut butter for dipping?
Toasted chickpeas as a snack? or veg / fruit crisps (not sure how nutritional the store bought ones are, but a suggestion)