Can my ex deduct medical if I have custody, claim them and pay half medical?

Only the person who claims the dependency can deduct medical bills paid.

This advice was prepared based on the tax laws in effect at the time it was written based on the facts that you provided. Click on my profile to read more. Errol Quinn Enrolled Agent Master Tax Advisor

For medical purposes ONLY, both parents can claim the same child. This allows them to list them as dependents with their jobs and claim the expense on their tax returns. (This was a new revenue proc 2008-48 last year.)

For actually claimint the child's exemption and child tax credit on the tax return, that is the custodial parent's right, unless they sign the form 8332.

http://www.irs.gov/irb/2008-36_IRB/ar08.html

No, he cannot deduct the medical because he is not claiming them as dependents.

You have to look at the wording of the final dissolution of marriage. Sometimes a judge will calculate medical expenses into the child support that is paid, but more often than not, it is a separate expense. If you ex is paying for the health insurance, that is calculated into the child support- the more he pays in health insurance for your child, the less he has to pay you in child support.