I agree with what Anvilhad said. He does not have long term sobriety and recovery and that puts the children in a bad place if they were to interact with him. The emotional disappointment alone is devastating to a child, when dad doesn't show up or gets arrested.
My son lost the rights to his son and also his daughter (by a different relationship). Both moms did this for the safety and well being of the children. I was not allowed to visit either because I was close to my son at the time (he has been missing, lost in his addiction somewhere for the past 10 years or more).
It broke my heart to lose my grandchildren, especially the boy who I knew well and had spent a lot of time with (the girl, I only saw once before her mother took her away). But as much as it hurt, I knew in my heart that the mothers had done the right thing and I took comfort that their children would not be hurt by addiction again.
I am sharing this so you can maybe understand that, as much as you love your son and want to see him happy, this is so much more than that and until and unless he can put together at least a year or more of complete sobriety and recovery then the mothers are doing the right thing to protect their children.
From this mama/grandma's heart to yours...hugs.