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I am very sorry to hear about this situation.
Is it possible that he can get sole custody and more time with the children while he is under pot?
It is possible, but just because it is possible does not mean he will get what he wants.
The Court decides on custody based on the rule of thumb of "best interest of the child." This includes, but is not limited to, general stability of the parent, financial stability, indoctrination of the child in the current school and environment, household condition and living condition of the child, other persons living in the house, etc. The courts generally do not like to split the custody 50/50 since this is hard on the child unless both parents agree to this.
Once custody is set, it can only be modified if the filing party can prove that (a) there has been a substantial change in circumstances and (b) the modification would be in the best interest of the child.
He is filing for sole custody, you say, meaning that he wants to have visitation and for you not to see the child? Unlikely. Even if a parent does not get managing custody, they are almost guaranteed visitation unless they have a drug problem, alcohol dependency, or an unsafe home environment. Abuse and or neglect of the child or previous children are an almost automatic bar for even visitation, although supervised visitation may be granted by the Court.
So far, there has been no proven evidence/facts from what you have told me that states that someone in his situation can show that you are a danger to the child. Ergo, chances of sole custody is unlikely.
TO MODIFY FOR HIM TO BE CUSTODIAN, he would have to show (a) there has been a substantial change in circumstances and (b) the modification would be in the best interest of the child.
IN THE UNLIKELY SCENARIO THAT HE COURT APPROVES FOR HIM TO BE CUSTODIAN, VISITATION CAN ONLY BE CUT IF he can show that you are a danger to the child.
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