Smoothies are great. One of my favorites is an organic blend of kale/spinach, nuts or nut butter, frozen berries, and water. It's ready in about 2-3 minutes.
I rarely prepare fresh juice, but take the easy way out by ordering it from a local health food store. I'll pick-up fresh fruits and veggies while there. I also use green and red powders, but that's just me.
Oatmeal is great for breakfast, and is a great alternative to sugary cereals. I always add berries. Hemp seeds have a nutty taste and can supply Omega 3, so I'll use those at times, or nuts.
Beans can be added to salads for an even greater nutritional punch.
Salads are versatile and great as long as they are not spoiled by adding calorie-laden liquid oils.
Nutritional yeast has a nutty flavor, is a good B12 source, and can be sprinkled on top of pop corn, salads, cooked veggies...
You can use a steam/rice/slow/ "whatever" cooker to quickly prepare meals. I sometimes use mine to cook beans or brown rice. At other times, I will make a really fast meal by steaming a potato, together with an assortment of greens, a beet, carrots, or corn-on- the-cob. I first steam the potato for a few minutes, and then add the veggies.
The steamer can be used to make soups, as well.
If your son really gets into vegetarianism and wants to have fun (if you have the time& interest), try sprouting. He might love the idea of growing some of his own food indoors. Check
YouTube for the easier-than-easy instructions. BTW, you don't need special equipment, a mason jar will do just fine.
If you want, a dehydrator can be used to make veggie or fruit chips.
Fresh fruit, or even organic dried white mulberries (I buy them in bulk from Amazon.com) are good for snacking.
Regarding supplementation, most medical nutritionists seem to recommend either daily, or weekly, B12 tablets for both vegans and vegetarians. They're really inexpensive.
Some also recommend an additional source of Omega 3. "Ovega 3" is one vegan option, and there are others. Alternately, and a wonderful option, you can use a coffee grinder to grind flax seeds in under 30 seconds; and then add them to cereals, salads, smoothies, etc.
I haven't run across any credible medical nutritionists who worry about protein intake on either a vegan or vegetarian diet, as you can get all you need from plants; and Americans actually have the opposite problem of consuming too much protein.