Perhaps he would marry Sophia of Holstein (born 1375), who IOTL married his uncle Bogusław VIII? Meanwhile one of daughters of Casimir of Słupsk from his Mazovian marriage could marry one of the sons of this guy:And who could Bogusław himself marry? Maybe Ricardis Catherine of Mecklenburg (OTL wife of John of Luxembourg)?
https://en.m.wikipedia.org/wiki/John_I,_Duke_of_Mecklenburg-Stargard