Read the Jessica Seinfeld cookbooks Deceptively Delicious and Doubly Delicious & then adapt the concepts. Since he doesn't seem to mind the purees, try adding sweet potato, butternut squash, or cauliflower to mac & chesse or as a spread inside grilled cheese sandwiches. Try adding red/yellow bell pepper puree to tomato soup & make grilled cheese dippers (fun for 4-year-olds). Make banana bread with 1/2 the sugar, cauliflower puree, and apple for 1/2 the fat. Make pumpkin bread with 1/2 the sugar.
It might be fun, too, for your son to pick out a new fruit each week to try - star fruit, lychees, pineapple might be a fun adventure for a 4-year old. And maybe if he gets to choose a veggie, he can try it in fun ways (bugs on a log celery, sweet potato fries (baked?), zucchini spaghetti?)
You could also keep your fruit smoothie/purees going - those all sound like great treats. If you get a really good blender, you could make them at home too, even adding in spinach, beets - really anything - as long as the fiber gets ground up well, and you have enough yogurt/fruit to cover up the bitter.
Also, know that these things change over time. I was extremely picky growing up - would pretty much only eat noodles with butter/parm, plain chicken or steak; I think my only fruit was apples with peanut butter, and my only veggie was celery, carrots, green beans or corn. Once I started high school, I started trying other foods that my friends were eating that seemed "grown up". I can't believe it, but I didn't even eat tacos, hamburgers, salads, until high school or college. Now I'm a really healthy eater, and am trying to instill in my kids a sense of adventure when it comes to food. Growing up, it was such a battle that I want to avoid. My parents would always force me to try everything on my plate. I wish they hadn't done this, because in my mind, I hated the way veggies smelled, and the texture of fruit grossed me out. And for me, once I had convinced myself I didn't like something, no amount of forcing me was going to change my mind. It was much later, when I thought - wow, pineapple smells good. Why don't I give it a try? THAT'S when my tastes expanded. I think it would have been better if my parents had said - OK. we'll keep serving it, and when you get a chance, maybe you'll want to try it.