I have just started my 2 year old in a copper lozenge loose ring snaffle but I know several people recommend a simple rubber straight bar. Ensure whatever you use is high enough in her mouth so she can't get her tongue over it as once they learn they can do that it may become a habit that is difficult to break
Straight bar rubber snaffle for mouthing, and then introduce a training bit a week or so before the start of her training to insure she is used to it before she starts.
I'd just look at his mouth confo and see what suits it. Always wary of rubber striaght bars are they're ofter too thick for horses mouths and are too easy to lean on
I got one of those straight but bendy rubber bits with the little brass rings. Suitable for yearling. Now mine is 2 I show him in a small eggbut bridoon bit.