My GS got it last summer. I think it took about 3 months to clear it. We were naive and there is alot to get your head round.
I assume your GS has been using Derbac M? That is what worked for GS, doctor prescribed Lyclear which he used several times and didn't work.
Really important that they treat at the same time. The day you treat strip bed and wash all bed clothes and towels on a hot wash. Have a shower or bath, make sure body is dry and apply the Derbac all over, you get told to avoid face and scalp but we did that in the end. You keep the Derbac on for 24 hrs, if you wash any off, say washing hands after using the toilet, you must reapply to that area. You wear all clean clothes after the treatment. Things that haven't been worn for at least 3 days or have had a hot wash. Anything not washable put it in a sealed plastic bag for 3 days. Make sure to vacuum well after treatment.
To make sure you get any hiding under nails use an old toothbrush to make sure the lotion goes under toe and finger nails.
One week later you do it all again.
The itching will go on for a while, maybe a month or more. The important thing is no burrows, the little threads where the eggs have been laid.
I bought some cheap trainers for him and all his shoes and trainers got sealed in plastic bags for a week or so to make sure anything on them was dead.
If you have a few treatments you can get irritation from the treatments so sometimes it seems hard to know if it is the irritation or scabies but again the big give away is if there are threads.
It is a horrible infestation and I think it can make you quite paranoid. I hope he gets it sorted. With GS I had to tell him to stop with the treatment and give his skin a chance to heal and fortunately that was it.