Hi Gunslinger and welcome to the forum.
As for your question, you should tell us how old your son is and what he/you would like to do with it. Furthermore I don't know if you play keys aswell, so I'll start at the beginning of the world of keyboards:
There are two existing kinds of electrical "keyboards".
One is the "keyboard" and the other one is the "synthesizer".
The difference between those two:
A keyboard is very good for playing alone, because all keyboards have a "rhythm machine", which means that a keyboard can accompain your playing with a full "band". You simply play the melody with your right hand and control the accompainiment with the left one. The disadvantage of keyboards is that they have a not too good sound quality and you cannot "program" them.
A synthesizer doesn't have a rhythm machine, but they are used in bands and you can prorgran sounds on your own.
However, I guess that you're searching for an "entertaining" keyboard. I'm not up to date in that world anymore, but a manufacture with is in MY opinion the best in refererence to keyboards is Yamaha. A manufacture which I would not propose is Casio, because they produce mostly keyoards for playing around just for fun.
If your son is still very young and you just want to find out if he has fun playing keyboard, you can really look for a new keyboard in the prize range below 200$, but if he is really looking forward to play keys for a long time and you can be sure that it is true, you could also try to search a better keyboard, which is used on ebay or something like that.
I just had another look on the keyboards shown on the page you gave us!
Another important thing: If you want your son to play piano with both hands (or something else without rhythm accompainiment), you should be sure to buy a keyboard with as many keys as possible!
However, you should give us more information about what you're looking for exactly and how old your son is, because the age could be important in that case...
I hope I have helped and not irritated you ;)