I have a horse that is extremely lazy. He is a five year-old Thoroughbred off the track but it's not someting chemical, we have had blood work done on him.
I have known him since he was a baby and has always gotten great treatment and training.
For a while I would heavily use my whip (at my trainers suggestion) when he wouldn't respnf dot my leg, it worked great until he had a nervous breakdown and bucked or refused to move when I even carried the whip. He over that now but he is now numb to it.
I practice extending the trot, walk, and canter daily as well as transitions but he has a numb side. He has energy, mos of the time once we canter a bit he'll be fine but how do you get him going at the trot if you can't rile him up at the cnater first? Help!!!

Here are the good points:
You have known the horse since he was baby and how he was treated.
You know that his blood is o.k.

The training of a racehorse obviously is a little different to that of a saddlehorse. Racehorses are trained to go from 0-100 because of adrenalin, the rider's crop - everything else is "holding back" by the reins.
Your horse needed to learn to understand what legpressure means. Because he didn't he was beaten with a whip.
He indeed must have had a nervous breakdown - imagine you are getting punished but you just haven't got the foggiest idea WHY!
He cracked it. You would have, too! And I would have - big time.

Ok. now what?
He must have a lovely nature as he seems to understand that he is allowed to go forward once he is "rolling".
Cantering was his "job" remember? That's why you need to get him going in canter before you have "forward" in trot.
The poor horse is "guessing" his way through - he sticks to what he knows he is allowed to do. You have to build up his confidence by schooling him with this in mind.
Go back to ground work for a little while. Show him what pressure on-off means on his side. When he yields make a fuss and tell him how wonderful he is.
Under saddle start to teach him leg yielding (going away from the leg....) - ask a ground person to help do the same as you did when yielding off your hand...- with you on him, giving the aids (leg on/off)
No whipping! It's his brain / his responses you need to train (explain) and reward when he does the right thing.
That is how he will learn. It's called "positive reinforcement".
Be patient, because he will always remember being beaten up for not understanding.
Do your canter work in between, as this is what he feels most secure with....

Btw. dummy spurs might have had a better result than whipping. You can try and introduce them if you are sure your leg is steady and you can use them only when needed - not accidently (that is very important!)or as punishment!
Good luck and I hope you will find a sympathetic trainer who understands your horse better.

I believe there are no "lazy" horses - Movement is their way to survive in the wild. Horses life because they move. Refusing to move is "giving up" and this is only the case if they feel being trapped or totally and utterly confused.
Good luck and let us know what's happening.
Your horse is lucky to have you as you are trying to do the right thing. All the best.