The KDX would get my vote. AT 5'5", he is too big to be comfortable for long on a mini bike, and the bigger you are, the harder it is to control a small bike (your knees get in the way of the bars). The KDX is an easy bike to ride, and as noted above, can easily be lowered to help the boy get his sea legs. The upside of a KDX is that as he improves, he still has a bike that will make him happy, and you don't have to take a big hit on another machine.
For my two son's, I always bought the most bike I thought I could get away with, so Ben was on an 80cc when he was 7. I just had to lower the bike enough so he could touch the ground, and then make new dog bones as he gain height. When it was time, I did the same with the 125, and tomorrow, he will transition to his first 250cc (since he has already put some time in on my 500, the 250 is not a problem at all). I'm kind of looking forward to riding a 250 again, as he and I will now be able to swap bikes on trail rides.
Good luck in your search.
Rick