The reason behind the uncertainty of Abraham's fate is due to the nature of his deportation.
Abraham was on Mechelen Convoy XVI, which departed from Belgium on 31 Oct 1942.
Before the train entered Germany, approximately 177 deportees managed to escape from the train. The train arrived at Auschwitz with over 800 deportees and between Convoy XVI and Convoy XVII (which departed the same day as Convoy XVI), 702 men were chosen for labor (per Auschwitz historian, Danuta Czech).
Abraham does not appear in any records at Auschwitz, but if he had made it to Auschwitz, he would have likely passed selection based on his age. Not everyone who passes selection has records at Auschwitz.
There are two major possibilities for Abraham's fate:
a) he may have been among the 177 prisoners who escaped from the train in Belgium, and either died during the escape, or at another time during the war in Belgium (69 of these escapees were recaptured at some point during the war, but Abraham was not among them).
b) he arrived with the convoy at Auschwitz and passed selection. He would have then likely perished sometime in late 1942 or early 1943 (but sometime later in always possible). There is a smaller chance that he passed selection at Auschwitz and was eventually transferred to another camp and thus perished there.
Yad Vashem does not report that this transport stopped at Cosel, so he avoided the same mystery that surrounds his father's fate.
It is possible that he arrived at Auschwitz and did not pass selection, but that is unlikely unless he was very ill or injured at arrival.