Hi, gee, eventing with a heavy horse at his age... it's probably lucky he hurt himself & got out of it! He's probably pretty close to mature now though. I would really like some more info before I can give you much at all. Eg. are you an experienced rider? Experienced trainer? What things do you want to do with him aside from drive? And as for him, has he been taught well how to yield to pressure? Ground drive? Lunge? Voice commands? How well do you think he knows all that? What are his 'bad manners' on the ground? What sort of a personality is he? Eg. is he a bossy, assertive type? Laid back, 'make me' type? Afraid of anything?
With any new horse I tend to spend a while just getting to know eachother, building a good, positive relationship first without asking too much of anything, and regardless of their level of training, I start out with the absolute basics & work my way up. That way, you can just skim through all the stuff that he's good at, reinforcing the behaviour and your relationship with him in the process, but you can also find any 'holes' in his training at the foundation level and teach him better, rather than starting with the 'high school' type stuff & trying to sort out probs there.
These days people tend to give more weight to 'manners' and training the horse well on the ground, but still, there are many who just overlook these bits, in favour of just getting them going under saddle. Therefore, bad 'manners' are still a very common problem. Can't blame the horse, it's just that they haven't learned or been taught effectively how to behave around people. Teaching manners is firstly a matter of working out what behaviours you want & don't want, so you can be consistent about making sure the 'good' behaviours work for the horse & the 'bad' behaviours never do. Unless it's a dangerous behaviour, most of the time I prefer to ignore, rather than punish 'wrong' stuff, but whether or not you do this, timing is extremely important and you need to punish/reinforce the behaviour *at the time it happens* or at worst, within 2 seconds max, so it's possible for the horse to connect cause & effect. And whether or not you punish the 'bad' stuff, it is way more important to ensure you reward/reinforce the 'good' stuff.