@Yinny Maybe try making them in different ways e.g I looovvve roasted cabbage or roasted peppers into a salad, Charred kale that kind of thing.
Also maybe try boiling them less so they retain colour and crunch.
Hide the extra vegetables, if you use a juicer all the vegetable “trash” freeze it then break off a couple pieces and chuck into soups and stews.
Try different dressing, I always make a salad dressing like a lemon vinaigrette cos it’s like 4/5 ingredients in a jar and shake shake, but a yoghurt dressing is healthy and can be customise with herbs and spices like chive, raw garlic, chilli powder a pinch of sugar and a squeeze of lime juice, healthy and quick.
How old is your son, perspective also makes the job easier, xyz makes you big and strong. Did you know that when iron man was your age he ate beets every day. Why do u think his suit is red. Iunno whatever works man.
Present your veg in other ways, so hummus is still veg, dip some peppers into it great.
Last but not least get them to help, doesn’t matter what age he is. The kid and I were cooking from he was at least 6, let him explore. Start by giving instructions such as mix this for me please, or hand me so and so. Food is tactile and he can taste the results of his hard work, that sense of pride associated with a dish he made makes them eat more. Also ask his opinion, “taste this, does it taste ok?” He may not know but he may feel proud that you value his opinion, he may even make something up but he will enjoy being asked.
Pick a dish you wanna make together like a quiche for example, make it vegetarian and get him to help. You may just need to supervise and give guidance here or there.
This was how I expanded my little cousins palate, he also loves to cook and eat the results he’s 12 now and eats everything. The one down side is he never wants a kids meal, he wants a steak too

brown on the outside and pink on the inside he says.
Hope these are helpful.