Ok. My 13 yr old son is pretty athletic - runs track and competitively swims, and rides his trek mountain bike to school. 3mi roundtrip most days. Last summer he went to a few youth triathlons in the northeast and did very well, placing in the top 3 or 4 each time. I felt "like a bad dad" when I watched him peddle his mountain bike while other kids zoomed past him on their road bikes. He looked like a grandma sitting upright while others passed him with the 'right technology' advantage.

The youth bike legs are only 4 miles long.

I'd like to get him a road bike so that he can be more competitive this summer but I'm not a biker and have no expertise. The goal would be to keep him "near" the top bikers so that he would be within striking distance at the start of the 1 or 2 mile runs.

The local bike store has an entry level Trek road bike for $800 which is, from my perspective, a crazy price to pay for 3 or 4 races this summer. I've learned of a nearby shop that has a very serious 'fitter' who will do the calculation on my son for $50 and apply that to a bike if I want to buy one. This same person will 'rent' me the correct bike at $50 a day.

My son is still growing so a year from now he may be 2 or 3 inches taller.

Preliminary questions:
1) will growing 2 or 3 inches and increasing his weight from 120 to 135 result in a 'new' bike no longer "fitting" him ?
2) if we rent a bike for his races, is it absolutely critical that we rent if for a few days before so that he can practice on it or should he be able to 'figure it out' by warming up for a few miles on race day morning.
3) assume we get 'fitted' for $50 and then go shopping online, should I be able to buy something "good enough" online ? Ebay shows aluminum road bikes for $200-$300 which might be good enough.

I know there is no right answer to the above but I've got to start somewhere.

Thanks.