Is he currently a high flyer on a BMX? or he just rides a bike? If all he wants to do is ride a bike and not do all sorts of stunts then go to Walmart and get a BMX there, you don't want to spend a lot of money because he's going to outgrow the bike in 2 years, just make sure you get the largest size that will just barely fit him so he can grow into it.
Now if you're thinking I'm some dumbarse for telling you to buy cheap then don't buy cheap, but don't go crazy either, now there is a bike put out by Elite called the Stealth Freestyle, not sure if it will fit your son but I think it should. It's not a top of the line bike, but again he will outgrow it.
A lot of bikes are difficult to get due to the C19 problem so you will have to do a lot of shopping around.