Good job knowing he needs them! Vitamins can't hurt, but the real nutrition comes from real food. Definitely be creative and hide them in food, but also, you can teach him to eat them. My step sister has 12 kids, some were picky eaters at first, this worked for all of them.
Make a green veggie in its natural form, but make it really good-spinach sauteed in a little olive oil & garlic, etc. Get the flavors as good as possible.
Cut it up in TINY tiny bits-not big sloppy glops, and put a miniscule amount, like one or two tiny baby bites on his plate.
Let him know he has to eat that before he eats the rest of his food (something he likes).
Make sure he is hungry before the meal-no snacks. Be super light hearted and happy about it, and celebrate if he takes a bite.
If he doesn't eat it, don't get upset or make a big deal of it. Don't coax too long or allow a power struggle, he just misses that meal. No snacks until the next meal.
Don't discipline or force him in regards to eating, but do enforce normal rules and table etiquette. Discipline for tantrums or throwing food. Don't let him rebel against your food prep duties!
It won't take long (usually one missed meal!) for him to eat the tiny bite of veggies. Over time, you can increase the portions. In the beginning, make ever new serving VERY TINY, and big bob of veggies is too scary to a picky eater.
This is if you want to draw the hard line, if you don't want to do that-disguising the veggies is the best way. Good luck!