Buying boots for yourself can be challenging, and when buying for someone else, particularly kids, even more so. The most important thing is to find a boot that fits your child’s foot well. No matter how good a ski boot is said to be, if it doesn’t fit the foot, it won’t work well on the slopes.
Given children’s growth rate, you can typically expect about two seasons max out of a kid’s boot. With that in mind, I believe it is okay to buy a boot that is a little bit bigger to begin with, so that there is some room to grow into it. However, make sure that it is not too big. Most important, whichever boot you pick for your boy, make sure that he does not slip around in it. If the boot is too big it will be an uncomfortable experience on the slopes. It is hard to ski in a lose boot, and it will be even harder to progress to the next level of skiing when there is constant slipping and sliding. As there are several different approaches to determine the best fit (at least theoretically), I strongly recommend going to a renowned ski store and have the knowledgeable staff assist you in picking the right size. Rule of thumb for a two season-fit is about an index finger’s width between the toes and the front of the liner. I have seen people shoot for more room, but I would recommend against it. Yes, more room will keep toes warmer on cold days, but too much room will make for a poor fit, and difficulty in maintaining control. Try to find the best compromise. See that the boot fits comfortably, without the foot being crunched in at any place, and that the heel is snugly held down in the back. Let him walk around the store a few rounds and let him communicate how things feel inside the boot.
Note that there are different boots for different skill levels. Generally, more advanced boots are stiffer so more energy can be transferred directly from boot to skis. Race boots are the stiffest boots you can buy, so unless your child wants to race, there is no reason to buy a race boot. A stiff boot can be very uncomfortable and restrictive in her flexing and extending abilities. Beginner boots are usually the softest, but I never recommend buying absolute beginner boots since junior kids usually progress quickly, and once they get to a higher skill level, a boot that is too soft can be problematic. This is especially true when they learn to do quick, reactive short turns, or want to get comfortable skiing on steeper slopes or at higher speeds.
The boots reviewed cover the middle ground, they have enough flex and are soft enough so even beginners can comfortably learn the sport but they offer enough stiffness to give advanced skiers the power transfer and control they need for their higher level skills.