You can file and claim the child, but unless you have his permission in writing, he could come back up to 3 years later to contest.

You also must file as "married, filing separately," rather than "head of household," which eliminates you from claiming the earned income credit, and other benefits.

It would probably be best for you to take whatever amount he might be entitled to the future, and bank it for over 3 years (or until the divorce is final, and things are settled) to be safe.