I can't be the only person to wonder about this... right? I mean they change their entire lower torso, anything worn directly on their legs or feet would come off when they shift, including armor. I suppose you could wear a shirt or kilt but I'm wondering if how Lapith deal with this is ever elaborated in lore anywhere.
This is an issue with any shapeshifter who can take significantly differing forms which wear things. As far as I know, there are no firm answers as to what happens to worn items when a creature stops being under a polymoph effect that radically alters it's body shape.
Also, Lapith are 3pp, and your question technically belongs in the 3pp forums.
I apologize I didn't know we had a forum for just Third Party Products, I suppose magic might also explain what happens to their lower body items, but it be fun as a challenge to/as players to work around.
Mechanically, since you are changing from a humanoid to a monstrous humanoid form, your clothing reshapes and resizes itself to accommodate your new form. You can generally handwave such things when using short duration polymorph spells.
However, a Lapith can stay in centaur form indefinitely, so he might prefer not to change forms while wearing shoes, underwear, trousers, or any other clothing items through which you insert your legs or feet when putting them on. While such items do adapt to the new form, I would assume that they would be slightly uncomfortable when altered by Formshift.
The most convenient sort of outfit for such purposes would be a skirt or kilt, as it would fairly conveniently reshape itself to cover the horse torso. Alternatively, he could simply remove all clothing from his horse part after he shifts and let that clothing revert to its normal form, assuming that he is comfortable with the idea of wearing only a shirt while in horse form (a cultural point that it is probably best left to the player to decide).