It sounds like he has made great improvements over a very short period of time. Good Job! What exactly is it you want help with? You said he is bolty, stiff and doesn't round. Quite frankly, I am not surprised- he's not there yet, either physically or mentally. Have you seen the training pyramid used in dressage (Training : Orion Dressage Sporthorses
has a picture), rhythm and relaxation (aka suppleness) are the basics of training and the base of making a good horse. That takes time, trust, more time, strength and training. A horse is not ready to move into anything past that, let alone being round, until he has those mastered.
How good is he at serpentines at the walk? At the trot? It's going to take him even longer to learn to be relaxed and soft than a youngster, because not only does he have some bad past experiences, but he is older. Therefore he is more likely to be both physically stiff and mentally resistant, and it is going to take him a while in regular work (think months, and more than three) to really build up his cardiovascular system, his muscles, tendons, ligaments and bones. Being rhythmic and supple takes a fair amount of strength and confidence. Give both of you a chance to actually get there. If you rush, you invite anxiety and more rushing on his part, which just equals a rushy, bolty, stiff horse. Master the basics before you move on. It's not glamorous to spend weeks and weeks walking and trotting, but worth it in the end.
Once he can do serpentines walking and trotting, while staying nice and rhythmic and relaxed and he is soft and giving to your aides, then maybe start thinking about working on contact. I don't have much advice about his canter, other than that, yes- a horse that is stiff at a trot will likely be even worse at a canter. I would consider doing lots of canter transitions once the trotting serpentines are going well.