Go to you're LBS and check out what kind of BMX's are in your price range and get them to throw in the following things:
2 bolt seat post.
2 Chain tensioners.
3 piece cranks.
I have yet to break my 1664 post and cause my seat to move, I'd imagine any other post like an Odyssey Intact would be the same. Chain tensioners will prevent the chain from becoming loose and when it does it makes it a lot easier to be set up again. The cranks are so you can get pedals with a 9/16 threading, a lot harder to break then the 1/2" pedals your sons bikes most likely had, plus they're just a lot stronger.
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"the bus came by and I got on, that's when it all began...there was Cowboy Neal at the wheel of a bus to never-ever land."