Just to clarify on this, OP. Blending fruit and veg doesn't take away nutrients. It's the industrial processing that takes away nutrients (pasteurisation, heating, adding chemicals etc).
When they talk about issues with blending, all it means is that blending fruit causes the sugar to release more quickly.
If you eat a whole apple, your body has to slowly break down all the fibre and pulp (basically do what the smoothie maker does), so the sugar is released slowly.
If you eat a pureed apple, all the blending is already done, so the sugar goes straight to your blood, and you have a sugar spike.
It doesn't mean that eating blended/ pureed fruit is always bad - but you just have to account for the sugar being a bit quicker. Blended fruit (not veg) should be seen more like a treat food.
If you blend your own fruit and veg it doesn't 'lose' any nutrients. All the vitamins will still be there. It's the industrial processing that takes away the actual vitamins.
Please don't worry about blending veg like spinach and carrots - they are not high sugar foods and they are perfectly fine to blend up in a smoothie. They won't lose the vitamins.
Just be mindful that if you put fruit in a smoothie, the sugar will get into your son's blood more quickly, so you might want to limit sugar over the rest of the day. It doesn't mean you shouldn't ever do it though, just see the blended fruit as a treat.