Commentary Purg VII 115-117

Dante refers, among Pedro's four sons, either to the firstborn, Alfonso III of Aragon, who reigned for six years (but not with happy result, according to the chroniclers) after his father's death (1285-91) and died at twenty-seven, or the last, Pedro, who did indeed die in his boyhood and was never put on.  However, the text would seem clearly to indicate a son who did not succeed his father on the throne.  Torraca (DDP Torraca.Purg.VII.115-120) makes a strong case for the unlikelihood of Dante's celebrating Alfonso, thus promoting the candidacy of Pedro (the 'Marcellus' of Aragon, as it were).  He has been followed by most commentators.