Hi,
There are many good starter bmx's on the market to choose from and everyone each has their favourites.
I think the first question you would need to answer is what type of "ride" you're son has or will have access to -where you are (i.e. park with ramps or halfpipe or without, flatland/street etc)
This would determine the strength of the frame needed which would lessen the choice somewhat, but there's still lots.
I would suggest that you go and look at some bikes in a bike shop to determine the size of the frame as how it feels is the best way imo to determine what's suitable. Somewhat like buying clothes! 🤗 until you know you're perfect size- try them on!
In this case it's expensive to make a guess!
Hope this helps.