What is the ruling concerning the obsessive thoughts about purity and impurity that afflicted a young man who was not religiously observant, then Allah blessed him with guidance, which led him to not pray at home and forced him to perform ghusl for every prayer?

Repentance is obligatory for neglecting prayer. One must also disregard all obsessive thoughts and not succumb to them, such as deeming bedding, clothes, phones, or anything one touches impure based merely on doubt. The fundamental principle for all things is purity. Furthermore, one must make up the prayers that were missed or were performed without proper purification.