Joint Custody Tax Question?

My ex and I have joint custody. I am the primary custodial parent. I have my son 313 nights of the year. I pay 63% of everything. The court order also states that " I shall be allowed to claim the minor child as dependent for purposes of filing any and all federal or state income tax returns for the taxabble year 2007 and all subsequent odd numbered years..." My question is that for 2008 taxable year my ex will claim our son as a dependant. Can I still claim head of house hold since i have him for more than 50% of the time and have a court order that states that i pay for more than 50% of the expenses? Can i still claim my portion of the child care expenses even if i can't claim my son as a dependent?