Will all depend upon what the terms of the agreement are that he signed…
If I recall correctly, the terms for my last car order did not guarantee a delivery date and so changes in delivery date would not likely have caused the seller to default on the agreement or not be able to otherwise meet their obligations… and there will also be something in the agreement about what happens if the buyer cancels…
OK, so that is the legal position, but I find it hard to believe, in this market, that the dealer cannot fill that slot with another order and refund your Dad as a goodwill gesture…
My last order was changed, by BMW, with the specification changes due to the chip shortages… In that case, it was a default as they seller could not supply the car as ordered… We did a deal on a. Replacement and moved the deposit over…
Edited to add - I just went back and had a look at my order form last time to check. In that order, they stated an "estimated delivery date" (so, note, not a fixed date) and the terms of sale were "The Seller will endeavour to deliver the Vehicle and any additional parts or accessories as agreed under the Contract within the time agreed or if no time has been specifically agreed, within a reasonable time."... There is some wriggle room by my reading and they would, I am sure, have claimed that no date was specifically agreed and so any delay would then fall to a test of "reasonableness"... Is a three month delay reasonable in the current climate...?
