Will he work in a snaffle?
Soapbox Moment: The problem, from my perspective, is that the rules call for consistent light contact with the bit. Western curb bits are not meant to be ridden with consistent light contact with the bit. So, to me, the only viable option for western dressage is a snaffle. End Soapbox Moment.
As far as any specific western alternative to the pelham, western bits are a bewildering array of infinite variation complicated further by the fact that one manufacturer’s name for a particular bit may be another manufacturer’s name for a different bit. 
Look for something with a jointed mouthpiece and relatively short shanks that are swept back a bit. Some manufacturers’ Argentine snaffles fit that description. I have used a Reinsman Smooth Sweet Iron Argentine Snaffle before and like it well enough, so you might give that a try.
And I’ll spare you the Soapbox speech about direct reining with a western curb bit. 