He certainly acts like a nice horse. He's just about 5 yrs old now, so I imagine he may fill out a bit. Personally I find him a little slab-sided and would like to see him deeper in heartgirth for the length of his legs - BUT I betcha he still has another couple years' growing ("width wise") to do. Love his apparent temperament and his gait looks smooth, he seems willing and eager to please, as well as good to work around.
A 'typical' Rocky/Kentucky Mountain/Mountain Pleasure Horse temperament is biddable and willing, calm, takes things in stride. They are very kind horses, not prone to crabbiness or irritating little habits. Excellent trail horses. Should have good bone for their height, as well as good feet. Personally I like this horse well enough from his video to check him out further.
Are you set on a Rocky? Or also open to the other mountain breeds - Mountain Pleasure and Kentucky Mountain Saddle Horse? All Rockies go back to one foundation stallion, "Ol Toby". The other registries are a bit more diverse, but still (should!) have the same typical disposition, build, abilities as the better-known Rocky. The chocolate color is also seen in all registries, and many are double or triple registered in all three.
In the Rockies especially (smaller gene pool) you must be sure they've screened for eye issues. These can happen in any breed, but have been predominant in the Rocky because of the lack of outside (breed) influence. Any responsible breeder/seller will be very upfront about this and offer information on eye clearances. An animal can be a carrier, but not affected - if you're looking for a gelding then it's no big deal, but I wouldn't buy a mare that was a carrier unless I was 100% never going to breed her. Just personal opinion.
In closing, these are AWESOME horses!!! Good luck with your search.