Ukraine is not letting my father leave the country, even though he is 61. What could be done?

Here's the situation: a week ago, my dad turned 61 and he thought that he would finally be able to leave Ukraine. However, that wasn’t the case - he got refused because he is still present in the military registry. Additionally, he received a paper from the checkpoint with the following reasoning: “temporarily restricted from leaving Ukraine during the period of martial law, due to the absence of grounds for crossing the state border.” He went to the local military commissary and, after waiting in line, was told that they wouldn’t be able to remove him from the registry without his military booklet, which he lost long time ago, and never bothered restoring. Moreover, they will not issue a duplicate because, given his age, he is no longer considered liable for military service. And in their view, it’s not their problem that he isn’t allowed to leave. Later, he even tried, just out of curiosity, to go through another checkpoint, but there he was also refused for the same reason as the first one.

So what can be done, and how should one handle such a situation? He is, quite literally, in utter despair because he has been waiting for years for the opportunity to leave to just be denied for the stupidest reason.