How does a patient with muscular dystrophy pray when he cannot find anyone to help him perform ablution or sit him facing the Qiblah, and perhaps he is in a state requiring a ritual bath (ghusl) or his clothes have become defiled? And does prayer become waived for him in this situation?

Prayer is not waived for a responsible individual as long as their mind is sound. Therefore, your adult brother must pray according to his condition. If he can perform ablution (wudu'), he should do so; otherwise, he should perform dry ablution (tayammum). If he is unable to do either, he should pray in his current state. Whatever conditions or pillars of prayer he is unable to fulfill are waived for him. If he finds someone to direct him towards the Qibla, he should do so; otherwise, he should pray in his current state. If he cannot pray standing or sitting, he should pray on his side, performing the remembrances (adhkar) of prayer inwardly, and gesturing for bowing (ruku') and prostration (sujud) if he is able. The point is that prayer is not waived for him; whatever conditions and pillars he is unable to fulfill are waived, and he must perform what he is capable of.