Two of my neighbors bought pianos for $200 and $400 respectively from other neighbors who were moving. Of course, they may have needed tuning service after they got them set up.
Why don’t you look through craigslist and e-bay (local-only) to find a piano for your son to practice on? Lots of people have pianos sitting and gathering dust.
Your son is very young and you don’t even know how long he will stick with this hobby. These items are heavy and require special handlers and larger trucks to move them in. Therefore, many people sell them instead of move them, if they are not used.
Better yet, just ask around your neighborhood. You would be surprised who has one that is not using it and would be willing to sell it. Then you won’t have to move it too far.