Which major European leader's son died in a Nazi concentration camp because his father refused to agree to a prisoner exchange between him and German field marshal?

During the Second World War, Stalin's eldest son, Yakov Dzhugashvili, was captured by the Germans in 1941. The Nazis offered to exchange Yakov for Field Marshal Friedrich Paulus, who had been captured by the Soviets after the Battle of Stalingrad in 1943. Stalin famously refused the exchange, allegedly saying "I will not exchange a marshal for a soldier". He later added: "All of them [Soviet POWs] are my sons".

Yakov remained a prisoner at Sachsenhausen concentration camp, where he died in 1943 in unclear circumstances - some accounts suggest he committed suicide by running into an electrified fence, while others argue he was killed.

https://en.wikipedia.org/wiki/Yakov_Dzhugashvili