I file SINGLE, however I live full time with my child and his mother. Since we are not technically married and she makes more money, she claims head of household and claims our son as her dependent.

Here is my question, through my employer I participate in a "Dependent Care Account" of which I contributed $5,000 last year. Can I claim the money I paid for child care on my taxes since my son is not truly my dependent however I paid for ALL of his child care in 2013 including the use of money from a dependent care account?

#1;correct. I guess under the tie break rule.

#2;It depends; The problem is that the IRS will not let both of you claim this credit. Only the custodial parent can claim it, even if that parent is not claiming the dependency. The custodial parent is the one the child spent the most nights with. Since 2011 had an odd number of days, only one of you was the custodial parent. That parent can claim ALL the child care costs, even though the money may have come form more than one person. If the person receiving the credit wants to personally reimburse half of it to the other parent, that is irrelevant to the IRS. However, under certain circumstances, you, as a noncustodial parent, can claim the child tax credit and claim the child as a dependent instead of having the custodial parent do so

THANK YOU so much for responding, however I think you misunderstood my situation. I live with the mother of my child (my fiance) so we live as if we are married and have for the past 4 years, but we are NOT married, just haven't gone through the process. Therefore only she is allowed to claim him as a dependent since we file separately. Since she is a highly compensated employee, she doesn't get to do a DCA, but I do. So I want to write off the fact that I pay for my child's day care, but I can't seem to do so because I don't claim him as a dependent. Does that make sense?

THANK YOU so much for responding, however I think you misunderstood my situation. I live with the mother of my child (my fiance) so we live as if we are married and have for the past 4 years, but we are NOT married, just haven't gone through the process. So neither of us are the "custodial parent", we both are FULL TIME. Only she is allowed to claim him as a dependent since we file separately. Since she is a highly compensated employee, she doesn't get to do a DCA, but I do. So I want to write off the fact that I pay for my child's day care, but I can't seem to do so because I don't claim him as a dependent. Does that make sense?

#1;Yiu are correct;the concept of custodial/noncustodial parent is just for a married couple.
That is what I meant; as you can see, In general, living with your girlfriend does not qualify you to file your tax return as a head of household even if you meet the requirements for claiming a dependent UNLESS the child is your qualifying person,i.e. child/step child ,niece or etc; your GF claiming the child as her dependent, a qualifying person, can file her return as HOH and you as said must file your return as single.ALSO you, as you said , youlive as if you are married and have for the past 4 years. . . but you are NOT married, just haven't gone through the process., are not legally married, so you can’t file your return as MFS but in this case you must file return as single.NO MFJ/MFS ( you can't file a joint return/separation - that's only for people who are legally married.) or No HOH as you can’t claim the child as your qualifying person.However, your GF can claim you as her dependent(as nonqualifying relative) as long as she meets the requirements. you'd have had to live with her ALL year.

#2;You may be able to claim the child and dependent care credit if you paid work-related expenses for the care of a qualifying individual. As a single filer, UNLESS the child is your dependent, you can’t claim CDC; A qualifying person is your dependent child age 12 or younger when the care was provided. So as loong as the person(your GF) who pays CDC exp and claims the exemption for the child is the only parent who can claim the child and dependent care credit.

NOTE;just fore reference,for married people, for example, if you are the custodial parent, you can claim the dependent care credit for the child even if you can’t claim the child’s exemption. If you are the non-custodial parent, you can’t claim the dependent care credit even if you can claim the child’s exemption.