When should I be filing for custody of my son?

His mother has full physical custody however we have shared legal custody. She is facing multiple DUI charges as well as associated charges in 2 states, MA and NH. She has 2 DUI charges in NH since December both while already being in probation in MA for probation violation from a previous DUI in 2013. Associated charges in NH are, conduct after, false report to a police officer, driving aver revocation x 2. I already have him 3 nights a week and she is refusing to tell me what is going on and if and when she is going to jail. I have been providing all of the driving for him (even though our current custody judgement states driving will be shared 50/50. I want what's best for my son and to try to protect him from as much emotional damage as possible from this. Please advise. Thank you so much!