I would have thought a 6 year old would fit a 24 inch wheeled bike with a small frame easily, unless he is extremely small. Kids grow quick so I would not bother with all the mods etc, look for a nice 24" bike. My youngest is 8 and has now just gone up to 26" wheels and he is so much more confident.
We alo own a norco b line with 24" wheels, but the fame is quite big and my 10 yr old rides this in he alps eventhough he rides a 26" hardtail.
We also owned a hotrock a1 FS and a hotrock a1 FSR, both good. The FSR has a tiny 12.5 inch frame and should fit your son. Full Susser that actually works for litte ones. Look out for a used hotrock with RST forks, these worked great, the sr suntour ones fitted to the newer bikes are a bit rubbish unless your son weighs 10 stone. We used both....
Look for a used FSR if you do a lot of descending, I think they have now been discontinued by spesh so hard to get hold of. We just sold ours when my 8 yr old moved up to 26".