Should a young man who used to masturbate, then repented, weakened, and returned to it, and now wishes to marry and has read that it may cause erectile dysfunction, inform his fiancée about this, or conceal it?

The young man must repent sincerely of this bad habit. Among the means of assistance for repentance is to hasten to marry. He should conceal his sin and not inform anyone about what he has fallen into. He should not pay attention to frightening obsessions about being unable to get rid of it, for getting rid of it is easy, Allah willing. It is not necessary that it will result in weakness after marriage. So, let him rely on Allah and be sincere in his repentance and not fear any harm, for repentance erases what came before it, and one who repents of a sin is like one who has no sin.