You need to approach your father gently and remind him of Quranic verses and Hadiths that warn against disobeying Allah and breaking fasting in Ramadan. If you cannot approach him directly, you may let him hear some recordings or send him a message via any means concerning this point as well as concerning the harms and damages of smoking. You may also leave before him some fatwas, booklets or books that talk about these issues.

Also, keep making supplication for him that Allah guides his heart to follow the truth.

Most importantly, never let this derive you to be harsh, rude or ungrateful to him so as not to fall in a grave sin.

Allah Almighty knows best.