Suggestions on improving responsiveness?

My boy is a rescued 4yo (extremely oversized being 31" and almost 95lbs), who is very smart, but a bit on the pokey side with commands. He can now pass his CGC, and we are waiting on the next official titling around next week. The person before me had the dog for 2 years, and during the time, used tons of negative reinforcement (beating him to a pulp for pooping in the house, getting on the 'wrong couch', not listening to commands etc). And because of the way he was taught the basic sit, lay, stay, he has an EXTREMELY laid back and "im only doing this because you'll probably hit me if i don't" attitude. I have taught him much more than this, and we just began taking weekly obedience classes with a local schutzhund trainer and doberman breeder. In the house, if I have food he is very responsive and will do things almost immediately. However, without food that he can visibly see/watch, and especially outdoors, he'll take plenty of time just to sit down (It is harder for him to sit down quickly with his awkward size, but thats probably just an excuse for me to feel better about it xD). He absolutely loves our daily off leash runs, and our recall is going very well so far, with nothing but positive reinforcement. He is also loves to play a game of "find it" in which I will give him a scent then hide the item somewhere for him to find it.

Now the problem is with the basic commands and leash walking (as I was not the one who taught these things). He just seems to HATE listening to me without a hunk of meat hanging in my hands while doing these basics. For instance, he was "taught" to "heel" by having the leash yanked hardly, so if you even barely snap the leash to get his attention, he will cower for a second then proceed prancing around as if nothing in the world bothers him In training, I use a prong collar (correctly) and he will only need the slightest touch to keep him near my side. Although he will do this, his expression is miserable to say the least since he has been taught that if he steps away he will only be yanked. We are trying to get his CD title within the next year, but I think it defeats the purpose of doing the obedience training, when he is miserable doing the basics.

I have also been training a rhodesian ridgeback mix puppy (only ~5 months old) to do everything almost perfectly, he will sit, down, stay, roll over, shake, come (and sit at my feet anxiously waiting another command), leave it, and walk near me without pulling a bit, and he doees everyone of the things I ask him quickly, responsively, and happily. I have used similar methods with both of the two amongst many other dogs, so I know this has to be more of what Zeus's previous owners did than my own doing. I keep hearing more and more of how poorly dobe's react to negative reinforcement, so I guess this what everyone meant :roflmao: . As he is my first doberboy, I am always looking for help!

So far I have worked on more and more positive reinforcement, and every other command except for sit, down, and heel have improved exceptionally. If anyone has any tips for "rehabilitating" this guy I would greatly appreciate it! He certainly seems to want a job, just not one he's been beat over. Also, he is learning the look command, but i absolutely can NOT get him to look at me when outside, even in a sit stay or down position, even with food, completely ignores it. Any suggestions on this huge novel I wrote would be greatly appreciated!! Thanks