For a 6yo, I wouldn't bother with anything but entry-level skis -- just to save yourself some money. Skis at this level are pretty much all the same except for the side-cut, which helps with the turn but it's not critically important for the progression. My 10yo who is a competent carver started with straight skis. All kids up to 6yo (and beyond, with the rare exceptions if you can find them) stay in the backseat. IMHO, diversity is always good when opportunity lends itself to it. It's like learning multiple languages at the same time. It's always tough in the beginning. But given time, everything can become second nature.
Now, when he turns 7 next year, you have a few options (especially if happens to be taller). You have (with some overlaps): Entry-level (same as above; majority of the market), intermediate all purpose (e.g. Bandit Jr, Atomic SX-9 Jr, etc.), intermediate-advanced all mountain (twin tips/mid fat -- mojo 65 jr), intermediate-advanced race-capable (Fischer Competition, Elan Integra), intermediate-advanced multi-event racer (Elan RCX, etc.) and lastly advanced-expert event specific racer (you know, all the ones that call themselves WC Jr. or the alike).