Thanks everyone for your comments (so far).

We went to the fitter and are renting a 51cm Felt bike for his summer races. My understanding is that it's a $2k bike and it'll cost us 10% of that to practice and race on during the summer. Each time we rent for a race we take it out a couple of days early for him to get used to it.

My son's 5'6" like the other poster's kid who is riding a 54 cm bike. Different bikes could be measured differently I suppose but at our shop they had Felts in all different sizes so nobody was trying to push a 51cm bike on me since we were renting. So I would think a 54cm one is just too high for your son and that the chirping pro there is correct.

And to Stadjer who wrote " Competitive kids need good stuff to give them a fighting chance, not the best stuff that gives them an edge.", I just want to say THANK YOU ! You hit the nail on the head. Brilliant and fits my son to the tee. His edge will be the run. His plan is to stay close to the leaders through the bike stage and then run them down the last 800 m.

Andy