I bought a used specialized Hotrock 20inch wheel for my son when he was 5. It was an upgrade from a bmx. He would ride trails with me in all over Dallas on the Hotrock. He also rode that little bike on trails in Bentonville – Slaughter pen and Coler bike preserve. He did not do any of the black trails cause dad would not let him. He would eat up smaller jump lines and any bit of flow. If your trails have any elevation change, gears will help. Blake took to them quickly and still calls them easy gears and fast gears. The fork on that bike was a heavy coil sprung 60mm travel. I disliked the fork, however, during rides he would frequently use all the travel.
I agree with bike nerd it is all about weight the ratio between the child and the bike weight. A suspension fork is meh unless you think they are going to shred hard. Otherwise a coil fork is just too heavy for what benefit you are getting (not sure about air forks for such a small rider). Tires are super important depending on what surface you are riding. I think you need to keep in mind the tire weight. I considered almost all on the bikes on your list before I went with a used option. I think they are all great options for young riders. One thing I would recommend is checking the stand over height with your sons measurements. Get one with a low stand over height and he will take to the bike much quicker.
My son rode the hot rock for 3 years I was sad to see him out grow it. It was a great investment. Used it cost $100 plus the upgrades I put into it and the memories are priceless.