I just found out that my ex has been claiming our son as a dependent on his income tax form. This is not true, he does not support him (son is 37!) My son has filed his taxes for 2007 and on form it asks "can you be claimed as a dependent on someone else's income tax? Of course he said no. What should I do about this, notify the IRS?

The IRS will catch up to this pretty quick. Your ex is allowed to claim your son as a dependent at age 37 if (a) he (your ex) provides at least half your son's support and (b) your son has less than $3400 income. Since your son filed his own taxes I assume his income is in excess of $3400, so your ex will be getting a nice greeting letter from the IRS.