Why is this formulation different from the similar sentence about years? After that one the person's name follows 'Do sal baad...' so here why can't one say 'Do mahine baad Aaamir Delhi jayega'?
Apparently it is correct to say 'do mehinon ke bad' and 'do mehinon bad' but people in general parlance just keep it 'do mehine ke bad' or 'do mehine bad'