I agree with Ridergirl, he already knows NH, he's a horse.
To answer your other questions, horses are a result of everything in their life. They are not like computers that have special buttons to do special things, you don't install it and its there forever. Unless this horse was owned by someone as the same touch, feel, responsiveness, and cues as you, then you can expect him to respond off your cues just as easily as a horse that you've had for a while. He may have been dulled down by needless squeezing on his sides, which would be why he would respond to a vocal cue that was consistant with what he was taught in the past. Its very easy to train him to respond to leg by using consistancy with your commands.
As far as connections go with horses, this is how I see it. I communicate with people regularly and talk to plenty of people every day. There are only a select few that I have a connection with, such as my husband, family members, and close friends, and that is fine with me. I see so many horses every day that it is the same with them, I don't expect sparks and fireworks with everyone, but I do expect communication.
Remember that your guy is 27 now. Some basic NH exercises would be perfectly fine for him, but keep in mind that his body is probably not what it once was. Excessive circling, extensive lateral work, constant disengagement, and jumping obstacles would not be in his best interest physically and this type of work should be kept at a minimum.
I personally hate riding my beginner shooling horse. He is so simple and doesn't have any fancy buttons like my personal gelding that I've had since I was a teenager. However, my gelding has trouble walking in a straight line with beginners on his back that are sending mixed signals. My dull, simple, schooling horse is always their favorite as he moves mindlessly around the ring allowing them to figure things out without taking ever shift of weight as a cue. I don't dare change that and as they are capable of moving up the horsey ladder, they learn more advanced things. I understand that you got this guy to get you more confident undersaddle. Start listening to what he can teach you, and don't worry so much about what you want to teach him.
The language of the horse isn't something that you learn from books or listening to people, its something that you learn from listening to horses. I took 4 years of Spanish in school and still wouldn't trust myself in a foreign country. My husband never took spanish in school, but has many spanish speaking people working for him. Guess which one of us can carry on a conversation? Him. He lives in the middle of it and had to do learn it to get by.
Horses are the same way, you can learn some stuff from books or what people tell you. But if you truly want to learn that language, listen to the horse and let it become essential for your everyday life with your horses. When you do it long enough, you never have to "do" NH, its your life because you are constantly communicating with your horses.