How do you know if Retaining your child is the right decision?

My son is in second grade, he just turnes 8 the end of May. He has struggled all year and last year (1st grade) with Reading and Math. The school put him in a special Reading and Math group for extra help and yet he is below grade level. Discussion now is that we hold him back a year? How do I know that this would be best? My oldest son was held back in first grade for Reading, now he is in fourth grade and getting A's & B's, I feel for him retaining helped. But each child is different.