Have a look in his mouth to assess his conformation first. A fat tongue and low palate usually mean a thinner mouthpiece will fit more comfortably, and usually either a french link or unjointed mouthpiece suits these types better, since they follow the line of the tongue better and reduce the likelihood of pinching.
More room means a thicker mouth might be a better choice, especially if the mouth is particularly sensitive.
A high palate and thick tongue would be better with a ported mouthpiece, which affords the tongue more room but can bang on a low palate.
Very sensitive bars mean trying a hanging cheek is sometimes a good idea, as it reduces pressure on the mouth and tongue.
How does the horse react to poll and nose pressure? If pressure on the poll is a good thing and he goes happily that way, a pelham might be a good bet - has less mouth action, more poll action, and offers great refinement.
You also need to see what material best suits the horse - some like steel, others like a warmer metal (maybe copper or sweet iron), still others don't like metal at all and go well in rubber or vulcanite. Steer clear of nickel - it's not very hardwearing, and can occasionally snap. It's ok when used as an alloy with other metals.
The saddle needs to be properly fitted - style really doesn't make a huge difference to most horses although lots tend to go better in treeless saddles. Not all treeless saddles fit all horses though.
Hope that's of some use!