A western pleasure horse is no different than any other horse. he needs to receive the proper vitamins, supplements, and nutrients based on the amount of excersize he is getting and his body type. As your yearling is just that... a yearling, he probably doesn't need any joint supplements for now. I would put him on a good multi-vitamin supplement with all the basics... like select II, 707 daily essentials, etc. You also don't want to load him up with grain at such a young age as it isn't healthy and having too much weight on a young horse can cause problems with their joints, legs, and feet.
I personally boot up all of my horses when I ride. If I am just doing some light riding or ground work (or if the weather is really crummy, or I am riding in an area with lots of stickers) I use a pair of Davis Splint Boots and bell boots. If I am doing more strenuous work or heavy lunging then i use either Professionals Choice SMB III's, Elites, Classic Equine Legacy Boots, or polo wraps (get a lesson in applying polo wraps if you never have before- you can hurt a horse using them incorrectly).
Mostly I would reccomend having him started as a 2 or 3 year old (depending on both physical and mental maturity) by a professional western pleasure trainer and then continuing taking lessons with him so you are both educated in the same manner. It will make both of you more successful in the long run.