If he was gonna start his own company I think he would've done it by now. My guess is he's going back to Es, maybe offered a partnership or something. Think of the reasons why anyone would switch:
1) Bad blood- Not a likely case considering the farewell video.
2)Offered a life changing amount of money- That's how Vans got ALex Olson, but I'm sure Koston doesn't need it.
3)Offered something else other than money- Possibly a partnership? Maybe. or certain privileges, whatever they may be.
4)Starting your own company- Sure its possible, but I think he would've done that by now.
5) Slowly easing into retirement- Highly doubt it. These guys don't just stop.
6)Too much stress- Filming for Fully Flared must have been hell at the end, but it's over now.