Ответчик по иску о взыскании алиментов заявил

The defendant in the claim for the alimony recovery stated that he doubted his paternity. The judge invited the defendant to file a counterclaim, but the defendant refused, citing a lack of funds to pay the state fee, medical examination, and the services of a lawyer.
The next day after a court decision was made to recover alimony, the defendant formalized his claims in the form of a counterclaim, but the judge refused to accept it, indicating that after the decision is made, a counterclaim is not allowed.
The defendant does not agree with the refusal, since the court decision on the recovery of alimony has not yet entered into legal force.
Who is right in this situation?
What are the conditions for the court to accept a counterclaim?

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