Is there something that I can claim on taxes if my ex lists our son as a dependent every year? He lives with her but I am active in his life and help out financially.

You could claim the child as a dependent but to claim the other credits the child would need to live with you.  You just enter him with his social security number.  You should make sure she agrees or if you both claim him there will be problems with the IRS.

She can claim EIC, Head of Household and the child and dependent care credit if  she is the custodial parent even if she doesn't claim the child. The child must live with you for more than 1/2 of the year to be the custodial parent. In working through the dependent interview you need to carefully answer several questions:

This is how it would be done for her:

• When asked if your child's other parent will claim him/her this year per legal agreement, answer yes (the other parent will claim the child).
• When asked if the child lives with you more than 1/2 of the year, answer yes.
• When asked if you have a custody agreement, divorce decree or other written agreement, answer yes (presumably, TurboTax treats Form 8332 as an "other written agreement."

In doing so, TurboTax will include him on your return for EIC, Head of Household if applicable and dependent care credit only.