Final payment will be due in less than a month. If you cancel after final payment you will lose some or all of your money. You can purchase "Cancel for any reason" insurance but that can be a bit expensive and will not give you a "full" refund.
If your father cannot set his schedule then I would wait until he can and hope there are still cabins available. Or you can book and if he can't go then he loses the money.
If the cruise was six months later I would tell you to book and lock in the price but with less than one month to go before final payment I would advise you to wait.