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Yes, you can file married separate and take our son as the deduction. That is assuming you have "primary" physical custody and not joint custody subject to alternating deductions.
One issue with filing separate is that you lose 50% of some deductions. You might also lose your standard deduction is your husband is filing itemized.
I won't be able to provide a more detailed answer until tomorrow. I will say you will lose your standard deduction if he files itemized, which means you will only have a single dependent deduction; and you would only get 50 percent of most credits (education, child, etc.). With a little quick lookup, I can put some dollar amounts to this, but not until sometime tomorrow.