In my custody agreement, I agreed to alternate years on claiming my son on taxes w/my ex. If he doesn't request an 8332 form, can I claim my son regardless of the year?

I agreed to alternating years on claiming my son on taxes with my ex husband.  However, my son is with me 90% of the time and my ex doesn't pay his part of our son's medical expenses. If he doesn't request the form 8332 from me nor do I sign the form, can I legally claim my son regardless of the year?