You have a complicated situation because blu is all open seating, no reservations, and he won’t be able to join you there.
Since you are in Aqua cabin, you actually do not have Reservations in the MDR, even though it looks like you do. There is a flaw in the booking system which applies to passengers and suites and passengers in Aqua which makes it look like they are booking specific reservation times, but in fact they are not. So even though it looks like you have booked the MDR, you really haven’t.
I have no idea why Celebrity can’t fix this. It is a well known issue and has been going on for years.
If you plan to forgo your table in blue and go to the main dining room instead, please coordinate that with the maître d’ of blu.
As to having your father eat earlier, see a maître d’ on your arrival on the ship and they should be able to work that out for him with no issues.