I don't want to express an opinion on any of the bikes mentioned. Not that there is anything wrong with them, rather it depends on the actual and specific riding conditions your son will be using the bike on. We have in my neck of the woods bike trails that cover the gammot of conditions, from smooth level pavement to rock strewn single track mountains..
As far as upgrading bikes. I have two (2) mid-80s steel frame bikes that I have owned since they were new, so long paid for. I decided to start riding again so I repaired one of them (new tires/tubes/chain/seat), complete teardown and upgrade with new brakes and aero levers. The frame is in very good condition and I have 1000s of miles on the bike. Did all of the work myself. But I can draw a direct comparison to the amount of money I put into my classic steel bike with a 10 year old, much nicer "craigs list" bike I purchased for $300
In short in my opinion and personal experience if budget is the thing find something used that exactly meets the needs rather than upgrade. If any of the bikes under consideration are good to go just figure the cost of new tires, chain and brake pads plus a tune up and then get riding. Must have accessories like helmet, gloves, trail tools and general bicycle bric-A-brac along with riding pants and so forth will cost more than probably what you will spend on any of the bikes under consideration.
But on the other hand once you accumulate all of the stuff the cyclist can have a life long pastime that is quite satisfying.