My ex is preventing me from seeing my son, of whom we have joint custody. Can I just go pick him up from school?
If there is a custody agreement in place stipulating that a party may have access to a child at certain times, they may only have access at those times and cannot take action to enforce their custody rights on their own. Instead of self-enforcing a custody order, a party will need to take proper legal steps to have the custody agreement enforced as written.
Enforcing Child Custody Rights
The first option you have is to hire a lawyer and have the lawyer write a letter to the child's mother. The fact that she is mad at the child's father or "doesn't trust him" is not relevant in whether she allows him access to the child or not. The court custody order has awarded some visitation rights to the father, and she has to comply with that court order despite her personal feelings. A lawyer can remind her of this, and let her know of the potential consequences if she continues to violate the custody order. Most often, when people receive a letter of this nature, they begin to comply with the legal custody agreement; knowing you have a lawyer and you are going to fight for your rights is enough to scare them straight.
If she does not begin to comply, then you can force compliance by asking a police officer to assist you. In other words, you may go to her house with a police escort and that police escort can compel her to provide you with visitation. This is generally only an option if your custody agreement very clearly delineates exactly when the child should be with you (in other words, "every other weekend" may not be too vague for the police to help you in this way.)
You may also take the mother of the child to court if she is not complying with the custody order. She can be held in contempt for her flagrant disregard of the court's rule, and she may be fined as a result or even have to spend some time in jail. Disregard of a custody order can also be grounds for changing the custody agreement, so taking her to court may result in your husband getting more custody rights if the court believes that an alteration to the original arrangement is in the child's best interests based on the mother's destructive behavior.
Getting Legal Help
Getting legal help is going to be the best thing to do here. A divorce lawyer can advise you on exactly what your custody rights are and how to go about enforcing them.