My ex husband and I split daycare cost 50/50. I claim our son this tax year. Do I use the total amount paid for daycare for the year or just my half? My ex can’t use the amt on his taxes bc he c

It’s a little more complicated than that. 

Only the custodial parent (the one who the child lived with the most nights) can claim child care credit even if that parent doesn’t claim the dependency.  In any case, the total amount of care expenses is used.  It doesn’t matter who paid it. 
So the custodial parent should give the other parent a signed form 8332 even though you have an agreement. Even though the other parent gets the dependency the custodial parent if qualified gets earned income credit and child care credit and if otherwise qualified also can file as Head of Household.