I think it will depend on your son's build which frame would fit him best, primarily the proportion of legs to torso length. What about stand over height?

I'd tend to go the other way, unless using the 54 cm would put the seat too far above the bars. I'm basing this opinion on my on experience of being in between a 56 and a 58 cm frame size. My solution was to replace the fork on a smaller frame with a fork with a longer steerer tube. I have long legs and a short torso. The ability to raise the bars and adjust the saddle/ bar height relationship allowed a nice fit.