Reliable studies have shown that, at a given age, children vary greatly in terms of their overall size, development of musculature, development of teeth and jaw, strength, and a broad spectrum of other physical and mental characteristics. No child can, or should, be expected to begin or continue playing an instrument that's uncomfortable to hold and to play. It's the very best way to discourage further interest. Just put yourself in the kid's place and go with common sense and opinions based on experience with a large number of kids. Don't try for "We want to get a horn he'll be able to use for years and 'grow into'." You want to make it as easy as possible for him to WANT to practice and to ENJOY playing. If that means getting an instrument that will need to be replaced in a couple of years, then do that. Luckily you're dealing with an instrument for which that is pretty easy to do.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)