I have a question, how to do it correctly (according to the laws of Ukraine)? Boy...

... 16 years old, should be 17 in the summer, has been in Poland since the beginning of the war, now wants to go to Ukraine to graduate from school and return to Poland to enter higher education here. Question: is it necessary to register for the military when moving to Ukraine? Will there be any problems with returning to Poland at the Ukrainian border? It will be there for Sunday - two no longer.