Oh yes, my DS1 loves smoothies!!
We use:
yogurt
frozen fruit (I like mango and wild blueberries)
banana and/or pear, avocado works too
baby spinach
hemp or rice protein powder
fish oil (for omegas, you can't even taste it)
psyllium fiber
almond milk and juice (pure if you want to use it, or just omit if you don't want him having juice yet)
Other healthy things you can put in them include flaxseed or flaxseed oil (for omegas), probiotics, or salba. Plus whatever fruits you like.
It might help to talk to a naturopath first though before adding a whole bunch of different supplements, just to make sure the ones you use are appropriate for children, kwim?