We adopted our sons at the ages of 3 and 4 so all those behaviors sound VERY familiar
Here is what we did with openness in regards to their former foster parents, and what you might consider - and yes, she IS one of his mothers, and will be a part of his life forever, so taking that into consideration is certainly important - but also, due to the neglect and her issues, you want to ensure that contact is not detrimental. We allowed all initial contact to be dictated by the boys because we knew WITHOUT a shadow of a doubt that the fmom would be supportive of their transition and of us as parents. So if the kids wanted to talk to her every second day, we called. Slowly, it decreased.
I would suggest that you offer contact with her monthly - for example, you send her a minimum of 3 pictures and an update letter once a month for the first year, and bimonthly for the second year, and every 3 months the third year, every 4 months the next 10 years
That way SHE knows he is ok, and she has the opportunity to respond in letter format the same. With letters, you also have the ability to screen for appropriate wording - which is far harder on the phone.
You might suggest direct contact at your discretion - possibly once a month (phone) or bi-yearly visits with specific guidelines. For example, a clean drug test. Or the visits supervised by a therapist.
I would also suggestt that you have a mechanism in place for negotiating change. Have her know that this is the MINIMUM you are willing to agree to, but are open to further discussion and he adjusts and heals, and she grows and changes.
Jen