Is it permissible for a son to marry his maternal aunt's daughter (his mother's sister's daughter) who was not breastfed with him, after he was breastfed by his maternal grandmother for more than six months before completing two years of breastfeeding?

Your son is not permitted to marry the daughter of his maternal aunt mentioned, because his maternal aunt has become his sister through suckling, due to his suckling from his maternal grandmother. He has also become a maternal uncle to the daughter of his maternal aunt through suckling. For the prohibition to apply, it is not a condition that they suckle at the same time; rather, suckling from the same breast is sufficient, even if the times differed.