You can certainly start training in a snaffle. I find its best. Better than throwing them on a bit they don't understand. Heres how I teach mine to neck rein.
There was a really great article in Western Horseman about teaching to neck rein. It was super informative and emphasizes going slow. The trainer recommends doing your neck rein training at the end of a session...like in a cool down. Heres how he explained it. (I've been using a similar method for years and it does work!)
Start two handed. Pick up your outside rein and lay it against the horses neck. Don't pull. With the outside rein *and* fingers, push to the inside.
If the horse responds, praise like he just won the olympics. If he doesnt, add some leg. If he still doesn't respond, gently pull the inside direct rein. As soon as the horse tips his nose, release the pressure and start back with the outside rein.
Neck reining can take a while for horses to learn. Some need refresher courses pretty frequently just to keep the neck reining tight.
Once you have him neck reining pretty well on a snaffle, it may be time to consider a curb. Wait for him to be ready though.