According to my divorce decree my ex husband can claim my son as a dependent for tax purposes only even though my son lives with me. My son is now 19, goes to college that I am paying for. Can my ex husband still claim him as a dependent on his tax return?

Depends what your divorce decree states. If it states that your ex can claim him, then he can, up until age 23 (if your son stays in school that long). I suggest you read the decree and see if there is anything as to how many years your ex can continue to claim him - for example, does it state that he can only do this until your son reaches age 19?