IMHO, I would stay away from (American) Tom Thumbs. It isn't a matter of harshness in this case so much as of poor balance and construction. Even excellent hands can confuse the horse with the ATT, because of the short shank/broken mouth construction. Too often they are mis-marketed as a transition bit or a "baby" curb - totally false. I won't say all horses hate them or are confused by them, as there's always the odd exception to any rule, but by and large an ATT isn't going to offer much in the way of clear communication.
In terms of what to do about bits, I'm with smrobs on this one - focus on getting him relaxed with his normal bit. If he's that nervous about the bit, changing the type, especially to a curb, isn't going to cure the problem. You might start with trying to rig him in a snaffle headstall (NO REINS) and turning him loose in a sand lot, roundpen, arena, etc. to just get used to wearing the bit without any "stress." Might be a good starting point.
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown