My mom's horse crosses his right hind leg over his left hind when walking?

Get in a GOOD farrier, one who specializes in corrective shoeing and can asses his gaits. Start there, it also sounds like he needs serious conditioning and nutritional program..if he cannot even pick up his right lead even on the ground on a lunge line then it could be contributed to his conformation or just his lack of muscles to sustain it, especially if hes never done it before, to him it will feel weird, thats where a ton of ground work will be very beneficial to him. After the Farrier, contact a good trainer.