Don't take any crap from him. Body language should always be 'quiet' but insistent. Develop a real 'good boy/bad boy' voice so he knows the difference. I never lose my temper with a horse but I do know how to shout to make my point if they start to piss me off too much
The join up that Monty Roberts uses is based on the 'prey/predator, cat and mouse theory that produces a horse that surrenders and surrender is begrudging not a willing act.
Hempfling makes some interesting observations on this on one of his Nature2promotions videos on youtube
Its better to use round pen work as a means towards the horse working with you respectfully as part of his education rather that racing him around until he's so exhausted he gives in
Treats are OK as a reward for good behaviour. You have to get the right balance of 'stick and carrot'
Beware of smoke and mirror tricks. Many of these 'trainers who you see doing miracles with join up arent working with genuine difficult horses at all, many have already spent hours running around that pen. At least Clinton starts out with a real problem but he is a very experienced dominant person and knows exactly where to be to keep out of trouble
If your horse doesn't respect you on the ground when you handle him then he isn't going to respect you in that round pen either and you are likely to get hurt if he doesn't see any point in playing a nice game.