Without knowing the small print on the back of the contract you signed, it is hard to know if you can cancel or not. But I would say a delay from June to September on a dedicated summer car is significant and would be a reasonable reason to cancel. If the delay is so long the factory has not started building it yet so there should be no financial loss.
When we ordered a car last year we put a firm delivery date in the contract and set out from the outset that if we did not get the car the dealer had the right to provide us with a replacement car until ours was ready - or we could cancel. And hey presto, our car was three months delayed during which we did more than 4000 free kms in a dealer car, incl. a trip to Denmark