Is the eldest son still responsible for performing the Qadha prayers of his deceased parents if they had not prayed all their lives due to lack of knowledge or negligence? Will they still get rewards for it?
The eldest son is responsible to perform the obligatory Qadha prayers of his father if it was missed due to a valid reason like illness etc. If the parents did not perform their obligatory prayers for no valid reason, the Qadha is not obligatory on the eldest son. Although, it is very good for him or other children to pay for persons who perform Qadha prayers on behalf of deceased persons to perform the obligatory Qadha prayers of the deceased. This will help the deceased a lot.
Wassalam.