My son is failing in school, the custodial parent doesn't make give him consequences.
The custodial parent has mental issues and puts me down in his presence and when I call the custodail parent does not know where he is half the time. The visitation is sporadic on her terms. Can I get physical custody in N.H.

Why don't you have custody of your son now? Usually the children are given to the mother and not the father. Depending on how long the child has been with the father the Judge may or may not change the custody arrangements. You will need to consult a good attorney and have good reasons for the custody change. It can be a long, expensive battle to win though.