Check to see if you can find a used model, you may be able to get a good deal that way. Craigslist and friends might be able to put you in touch with someone looking to sell a good 20" bike.

It is a tough call on what to do. It does not sound like he has younger siblings that could inherit the bike which makes that bite more palatable. If your son really likes riding then that extra bit will be worth it if the quality is there. Test ride the bikes yourself and see what you think between them. I have a slightly smaller frame but I test ride all my kids bikes from tricycles on up. I am not always happy with what they are but I know what I am getting.