Hi,
Which bit? I'd choose an unjointed or double jointed(french link) type. Rubber is the usual pref for youngsters, but so long as it's comfortable - right size etc, shouldn't matter. Some like keys, so their horses have something to play with. I don't personally like the idea - don't think they need more reasons to fidget with the bit, more reasons not to be concentrating on other things.
I think the more important consideration is what you do with the horse when he's wearing the bit. I would make a point of doing nothing much for the first few sessions at least, just allowing him to wear it & get used to the feel without being bugged, then choosing a time when he's holding it quietly to remove it.
After the first few sessions, I would start doing stuff with the horse, but NOT with anything connected to the bit. Pref. free, but if you need to be attached, use a halter. I'd be concentrating on only asking for easy stuff that the horse enjoys, make it all a good experience, try to keep him preoccupied while he's wearing it, so it just becomes natural to him.
Once he's accustomed to wearing it, then you can start to teach him to yield to the feel, but I strongly advise teaching him to yield to rein pressure in just a halter first(less chance of hurting him when you come to do it with the bit). Once he's reliably yielding in a halter, then start the same lessons in the bit. When you come to ride him, I would be doing the same there too - starting in a halter, to lower the chance of hurting him if he gets reactive to being ridden.