I've noticed in articles, and posts here and elsewhere that measured velocities on most bottle-necked calibers seem to differ about 25 - 35 fps per inch of barrel length change. Not my data, so use it for what it's worth. That of course will vary some depending on cartridge, barrel quality, load, etc. YMMV.
Accuracy itself isn't really affected by velocity per se, although a given gun will like a certain powder/bullet/brass/primer/OAL combo within a given velocity/pressure, etc. A shorter bbl. should act stiffer for what it's worth, and when shooting at longer distances, the bullet's path of a slower load can be affected more so by wind, misjudged distance, etc. when compared to a faster one.
My opinion - Give him a gun that fits him and he can carry and shoot well in a caliber that he can handle and yet will do the job. He can upgrade later - we all buy more guns later anyhow...makes the world go round. I can still remember being about 65 pounds, and being handed a full-size 12 gauge....I still fight a flinch, and that gun never did allow me to a d**n thing! Too big, to heavy and never pointed where I was looking. JMHO.