How mature is he? For the youngest boys, I suggest not more than a Victorinox Classic or Rally. I personally prefer the Rally with a phillips screwdriver instead of the scissors. A good alternative that is a better knife by far is the "My First Opinel." It is a better knife, but not necessarily a better gift for someone who might appreciate the other gadgets on the SAK.
For a more mature boy, say one who is closer to double-digit ages or who has demonstrated competence with a knife like those I just mentioned, there are innumerable fixed and folding blades about three inches in length which is an ideal size until their hands grow to their full-size. And to be sure, none of the knives I've mentioned are ever totally outgrown.
My personal favorites for 2 to 3" folders have been classic Case slip-joints, but there are a number of good intermediate size SAK's that are very good too, like the Pioneer. As pocket knives, these are fine tools, but they come short for dressing game or processing wood. A fixed blade has obvious advantages, but unfortunately, many states' legislation creates problems for anyone willing to conceal a fixed blade. As an EDC item, fixed blades can be legally troublesome. But a Morakniv is a good one for most any purpose. Fox River maybe for game in particular. The Victorinox Hunter looks like a fine knife for this. I was coveting one of those or one of the newer Buck 110's in SV30. In my mind, these blades are a bit big for everyday purposes. I carry a small slip-joint and it's pen-knife blade is the most used. I also carry a bigger knife, but it's rarely used. Wouldn't want to be without it though.