Is the 457 better enough to forgo having two calibers?
The question would be: Once you have your 455 22LR barrel scoped and sighted in for your son, do you really want to remove the stock, remove the 22LR barrel, insert the 17HMR barrel, re-attach the stock, and re-zero? Now, if you can keep good records on your scope settings for both, that re-zeroing could be reduced. The other question would be: Is it your son's gun (eventually) or just the one he gets to use?
Note that the CZ 22LR and 17HMR magazines are not interchangeable. I think you only get one five round mag of each in the combo. Additional CZ mags are pricey for what they are: a polymer 5 or 10 rd mag (they do make steel ones also). If you're going to spend your time shooting instead of constantly reloading mags, you're going to want to buy extras. Shooting five rounds, reloading five rounds, and repeating is not that fun. Having a few loaded up can be. The point is that I'm not sure that I would want to have drop another $150+ to buy extras of both 22LR and 17HMR mags if I didn't have to. Consolidation would be less expensive.
I’m leaning toward the 17 HMR for its extended distance, and accuracy. Any reason I shouldn’t?
Extended range is a given, but extended accuracy is not a given. 17HMR is still rimfire and can have some wild extreme spread variation. It is not rare for one box of 50 cartridges to have a spread of 200+ fps.This is compensated for by having a more uniform bullet (compared to 22LR).
On the quietest of days, 17HMR is a lazer. Throw wind at it and it is not quite as accurate at extended ranges. The good thing is that most squirrel hunting is typically less than 50yds. The bad thing is that if you want to eat the squirrel, you're basically relegated to FMJ or SP ammo. Vmax tipped or JHP ammo will have a high degree of fragmentation, unlike 22LR.
Another thought is the safety location and operation. If you're using this rifle to teach your son, the 455 has the safety on the bolt (which is different and not exactly intuitive) and it does take strength to operate. The 457 has it in a traditional location, with a simple lever operation.
At the end, I would probably still get the 455 combo over the 457 22LR since there is more value to the combo in an extra barrel and mag. CZ has basically cost cut in some way every successive generation of rimfires (for the base models). For squirrel hunting, I would probably just run an accurate high velocity load of your choice instead of a barrel swap. They are just squirrels. Hunters should be able to get within 50 yards. 22LR will still take them at 100yds. It's just that the 17hmr can extend to 150 yds or so.