I think the best thing you can do is keep exposing him to the trial environment and always reward him.
That's basically the key thing.
For us it's been a combination of two things:
(1) repeated exposures to trial environments, with lots of rewards/reinforcements for good performances (and what counts as "good" may be a very small thing in the beginning, depending on just how much your dog is prone to falling apart; with Pongu, who is an extreme case, I used to reward him just for walking into trial venues and being able to look away from all the terrifying things around him to make eye contact with me).
punishments, however mild, for failures. Not even a frown or a very mild verbal "nope." There is never any penalty associated with the trial environment, because that is already a huge enough stressor on its own. I do everything I can to make it a good place in the dog's mind.
With Pongu, I just resigned myself to the inevitability of repeat failures when trialing. We fail over and over and over, because Crazy Dog Is Crazy and that's how it's gonna be, nothing I can do about it. He fails if there's rain falling on the roof or a ceiling fan overhead or flies buzzing around the course. It's frustrating, but that's the dog I have, so I just aim to "fail upward" and do slightly better each time.
It's been about a year and we do pretty well now (sometimes, at least), but we climbed a mountain of NQs to get there and we'll have many more ahead. Again, I just try to accept this with Zen tranquillity. (I fail at that too. But I try!)
(2) practicing and proofing for the trial environment. Because I'm a reward-based trainer, what this means for us is that I have to get my dog accustomed to performing for long stretches of time without obvious reward. In the obedience ring, he has to get used to working not only without cookies, but without a lot of verbal praise or petting either.
If you don't train and proof for that, a weaker-nerved dog will often fall apart because they aren't getting the reassurance and support that they're used to, and that can make them start worrying, either about the environment or that they're doing something wrong (because you don't seem as visibly pleased as usual) or both.
There are whole seminars on exactly how you accomplish this, but in general it's a combination of environmental proofing and thinning down reinforcement rates and building up the inherent value of the exercises so that the dog finds them intrinsically reinforcing to perform.