I don't think you will go wrong with either a Kx250f, yz250f, rmz250f or the crf250r. I don't think much of the ktm and they have an issue with the suspension, but basically unless your son is racing at a optimum level and that little bit extra may make a difference in lap time by a couple of seconds, then you should concentrate on getting the best deal on a bike that you can. Find out who offers the best after sales service.
I would suggest that your son sits on a the suzuki as they are reported to feel the smaller of the 250f's and your son is resonably light in weight which can make a difference to throwing the bike around. My youngest races a crf250r and at 49.5kilos or around 105 pounds the bike really needs to be set up for him as it is a lot of bike for a lightweight,
So whichever you buy invest in a proper suspension set up as it really makes a difference.