Re: Correct canter lead
I think it sounds like you're doing the right sort of thing...the pessoa really helped all of my horses.
It might be worth doing some balance work with no stirrups in walk and trot if you think he'll behave. I know that I out more weight through my right hand side, and I am also heavier in my right hand and so this sometimes 'blocks' my horses from picking u pthe correct canter lead. If you think about it....to strike off on the right rein, the horse has to lift through his inside shoulder and he can't do that if your weight is there! One tip given to me was to take a quick glance over my OUTSIDE shoulder as I asked for the strike off...it places your weight more accuratley in the correct place. Worked for me!
Sounds to me though that one of your biggest problems is getting him forward....It's really important to make sure he is forward and responds immediately to your aids,otherwise you have this to contend with too...if you have to kick kick kick to get him to canter, you are much more likely to get the wrong lead as you'll both be off balance! You say he is overweight and unfit - it might just take him a bit longer to fitten up andthen maybe he will improve.
When you work him on the pessoa, give him a definite 'and canter' in an authoriative voice.... If he doesn't go IMMEDIATELY, get after him with the lunge whip!! Keep it short and sweet though as lunging is hard work for them! When he canters from this, you can transfer this spoken aid to your riding.