I would say for Polaris it would be an 07 or 08 700, For Cat 08 800 and for Doo it would also be the 06 800. Just from what I have seen these 3 sleds run all day with very few if any problems.
i have a 06 m7, 07 m8,08 m10 11 m8, i havent had the engines opened once, frames are tough, just add oil and gas and ride, ive pulled my share of doos and dragons out of the bush, i did rip the bumper of my m10 pullin a doo out (07 800 blown crank) and he bought me a new bd bumper.
the King Cat is a battle wagon! Point and shoot it is a great sled, lots of power. Other than that it sucks, twice as much effort to ride as any other ride forward sled.
I would go with the m-series since he has already rode the f-5 and has gotten used to that chassis. I think you could find a nice m-7 for around 4k. They are great machines. Very reliable and are very easy to ride. I had a F-6 and sold it for a summit 800 to go out west. The summit has great power but it was not as comfortable for me to ride. (harder to get it to lay-over and uncomfortable seat for me) I wish I could have found a nice M7. Good luck
If hes a pure boondocker go with the M8, bulletproof sled and handles very well. If he likes to do some jumping go with the 06 summit 800 in a 144, just change out the 13 wide chain for HYVO gears and chain and youll never have a problem, unless you tattoo a stump in the right spot and fold up the nun. The newer IQ polaris RMKs are great choices as well in the 6 and 700.
If your son get's into the sport and becomes a builder/modifier i would recommend an M series not because that's what i ride it's just a good chassis for building on top of....after ridding a KX 250 from the age of 12 there the most like a dirt bike and you can toe ride the sled, you don't have to muscle it. Also, the chassis are the same for the most part from 2005 to 2010 witch makes a difference in parts and availability of the parts he will need. Let's face it this sport is one that requires alot of parts and investment to do it right...and he will brake thing's. as far a motors... as your son gets more into building you can big-bore and or supercharge the sled and have way more power and performance than a stock M1000 without the weight of one.
Laters