Sober is an excellent place to be.
We have been on the roller coaster many times with our son so we knew that by him staying in our home he was not able to maintain his sobriety. He would do good for some time and then go back to using and it was destroying our life. He was taking us down with him and we were doing more work than him. We don't understand why this kept happening but it did. He wasn't as committed as he needed to be.
Now that he is away from us he knows he has himself to count on and that it helping him mature and figure out how things work in the adult world. We do maintain contact and provide emotional support and limited financial support.
If I KNEW he'd stay sober and live in our home and follow our rules I'd do it in a heartbeat but I am no longer willing to take that chance. It just never worked for us after several years of trying, so we knew we had to try a different approach.
I do hope that your grandson takes this great opportunity and support that you are giving him to grow and find his path. That is all that any of us want for our young adults and there are many different paths that we take to get to where we need to be.