This is so hard on you, but you must put yourself first. Also, trust the authorities to make the right decisions as to his welfare - we all know that mistakes happen but they are the exception rather than the rule, and it is a waste of energy to worry about it (easier said than done).
It sounds to me as though your daughter would not be able to cope with her son, and, unless he is very special, neither would her boyfriend. It might be that unless and until she has him back - for a weekend or longer, she will never accept this. Much as it might upset GS In the short term, in the end this may have to happen.
Also, somehow your daughter needs to get the message that if it was not you looking after him, it would be someone else- would she prefer that? If there is a real or imagined hatred between you, that is the worst thing for your GS as he will be aware of it and torn between the two people he loves.
Finally, and don't hate me or get upset for my saying this, when you worry, it sometimes helps to face your worst fears - if he goes back to your daughter and she does not cope, you will still be around to pick up the pieces and it will be much harder or impossible after that for her to get him back; alternatively, he could go to someone else - my understanding is that there are foster parents who are very good and experienced with autistic children, and as long as you had a good access arrangement, which you surely would given your involvement, he could actually thrive in a neutral atmosphere, and maybe you and your daughter could mend your relationship. You may never approve if her lifestyle, but if she loves her son and is not intentionally cruel to him, it could be worse. It would be so much better for him if you were both on the same side.
Please take advice and talk things over with an expert if you can, we, or certainly me, have not walked in your shoes. I hope it all goes well for all of you.
As I say, this is a worst case scenario and it may not come to that, but it might be a comfort for you to come to terms with the possibility instead of dreading it.