Philippe Bianchi told France Info radio his son, who remains in a coma, had shown "no significant progress" since crashing into a recovery vehicle last October at Suzuka.
"In general, progress needs to be made in the first six months," Bianchi said.
"It has been nine months now and Jules has still not woken up."
Frenchman Jules Bianchi, 25, continues to receive treatment for his severe injuries in hospital in his home city of Nice.
Philippe Bianchi added: "As time goes by, I am less optimistic than I might have been two or three months after the accident when we could have hoped for better progress."
Bianchi admitted that if Jules did awake from his coma, it could lead to added difficulties.
He revealed that, prior to his son's accident, the two had spoken about the condition of seven-time Formula 1 world champion Michael Schumacher, who suffered severe head injuries in a skiing accident in December 2013 and faces a long road to recovery.
Philippe Bianchi said: "If he had a severe handicap, we are convinced that is not what Jules would want.
"We talked about it. He discussed with us that if one day he had an accident like that of Michael Schumacher, that even if his only handicap was not being able to drive, he would have a lot of difficulty living. Because it was his life."
Jules Bianchi joined Marussia in 2013 and raced in 34 grands prix, scoring two championship points in last year's Monaco Grand Prix.
Marussia came close to collapse over the winter but were rescued and are racing as Manor in 2015.

The decommissioned Type 22 frigates HMS Cumberland, HMS Campbeltown, HMS Chatham and HMS Cornwall are currently moored in Portsmouth Harbour.
Bidders had until 23 January to register an interest in the former Devonport-based ships.
The BBC understands no proposals to preserve the ships have been submitted.
Those who have registered an interest are finalising their bids with viewings set to take place in late February and March.
A final decision is not expected until the spring.
The government's Disposal Services Authority, which is handling the sale, wants to award at least one of the frigates to a UK ship recycler to determine the capacity of the UK's industry in the field.
Penny Mordaunt, Conservative MP for Portsmouth North, said it was important UK recyclers had the chance to prove themselves in the field but she was also keen to see at least one of them saved from the scrapyard.
She added: "For anyone that has served on a ship it's your home, you've literally been through the wars with it... and you want them to have a noble second life.
"My preference is to go for the reef and diving attraction.
"We've got to get best value for the budget but a reef would also generate income for part of the country through tourism."
The Ministry of Defence has previously said it will "consider all options" for the frigates to ensure "best financial return for the taxpayer".
A spokeswoman would not comment on the number or nature of the bids received due to "commercial sensitivity".
Originally designed as a specialist anti-submarine ship, the Type 22 frigate evolved into a powerful surface combatant with substantial anti-surface, anti-submarine and anti-aircraft weapons systems.
They were also known for having excellent command and control, and communication facilities, making them ideal flagships on deployments, with a complement of about 280 crew.
Last year, the aircraft carrier HMS Ark Royal was sold as scrap for Â£3m.
Jules Bianchi's father says he is "less optimistic" that his son will recover from head injuries sustained in a crash at last year's Japanese Grand Prix.
[NEXT_CONCEPT]
More than 20 parties have come forward with bids to either recycle four Royal Navy frigates or turn some of them into artificial reefs, the BBC has learned.