Hmm...I recremend trying a few snaffles; some are better for your horse than others, you'll just have to find the "one" (sounds like a love story!). But just remember that some bits (pelhams, kimberwicks, etc.) are not aloud in dressage, prettty much just snaffles.
Anywhom, when asking him for a lengthening, post a bit higher then normal, down a diagonal, or a "long side". Push with your seat and squeeze with your calfs. Don't let him "run" but look down (I know, so bad, but a few times won't hurt) to see if you can see his toes flick a bit, or even better, try to feel it. It's a lot better to ask a trainer for help, since it's harder with people telling you over the internet and not in person.