All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
First of all, we ask Allah to cure your son from what he suffers, heal him fully, and guide him to the Straight Path. We advise you to supplicate Allah for him, as much as possible, because the supplication of a parent for his child is answered by Allah.
Abu Hurayrah narrated that the Prophet said: “There are three supplications that will undoubtedly be answered (by Allah): The supplication of a victim of injustice, the supplication of a traveler, and the supplication of a parent for his children.” [Ibn Maajah]
There are some signs to recognize magic. If you predominantly believe that a magic spell was done to your son, then the best means of treatment is Ruqyah. In case he does not accept you to recite on him directly, then you may recite on water and then give it to him to drink, for example; or that you recite on him while he is asleep. It is not a condition in Ruqyah that the patient knows about it. A Ruqyah could be performed on a person while he does not know. This is something experienced with mad people; they generally do not realize the Ruqyah that is done to them; yet, Allah cures them after the Ruqyah.
One of the most serious matters that you mentioned about him is his deviation from the Straight Path and his abandonment of the prayer.
Hence, you should seek the help of his righteous friends to advise him in a wise and gentle manner, and clarify to him the seriousness of the matter, and that the fear of Allah and preserving the prayer help achieve one’s aspirations, and that sins may be a cause of tribulations and deprivation.
Allah Says (what means): {And whatever strikes you of disaster - it is for what your hands have earned; but He pardons much.} [Quran 42:30]
They may also try to persuade him to look for another girl to marry and help him in this regard. If this is made possible, then praise be to Allah. Otherwise, if there is nothing that prevents that he marries that girl, then he should not be prevented from marrying her, especially that it is proven that Ibn ‘Abbaas narrated that the Prophet said: “The best solution for those who love each other is to marry.” [Ibn Maajah]
Finally, it should be mentioned that it is not permissible to accuse that girl of having done magic to your son without evidence, as it is forbidden to think evil of a Muslim.
Allah Says (what means): {O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin.} [Quran 49:12]
For more benefit, please refer to Fatwa 323014.
Allah knows best.