Some children are born perfectionists. My older daughter (now 9) is that way. She doesn't care if WE want her to be perfect, SHE wants to be perfect. She has an idea in her head of how she wants things to go, and if it doesn't go that way, she is upset with herself. If we tell her, "Oh, it doesn't matter if you're perfect, we love you just the way you are!" she just feels invalidated, like we don't understand what's important to her. She needs us to say, "What are you having trouble understanding so you can do it better next time?" - THEN she feels like we get it.
You may enjoy reading the book "Bounce: Mozart, Federer, Picasso, Beckham, and the Science of Success" by Matthew Syed. He studied exactly how all of these notorious perfectionists/experts in their fields became so good at what they do. Bottom line: tons of practice, expert coaching, and a love of what they do. The science shows that everyone (everyone!) can be excellent at any chosen thing (whether it's math, soccer, playing the piano, driving, you name it) as long as they practice (and practice and practice, mindfully and purposefully), and have good coaching. This is helpful to keep in mind when you have a perfectionist child. They already have the willingness and drive to be great at what they do. You just need to point them in the right direction and provide them with the coaching they need. Kids like this often times LOVE to be pushed (academically, socially, in sports, the arts, etc) - so go ahead and push them. They work best and learn the most when they work at the upper limits of their abilities.
You are doing well with you son, I think - praise his effort and then show him what he needs to do to perform the task better next time. Have him practice on a piece of paper that is not important (so he knows it's for practice, not the "real thing"). When his Valentine box is done, don't say, "Wow, that looks great! It's perfect!" Instead, say, "Wow! You really worked hard on that. Isn't it neat to see what your hard work can accomplish?" He will begin to appreciate the process instead of the outcome. Obviously to him, the outcome will always be important, as it should be, but he will understand that he has to go through a process to get to the outcome he wants. :)