Question 196: If a person reverts to Islam and they were unaware of fasting during their first Ramadan and missed the obligatory fasting, what should be done? Should they make up the fasts (qada) and pay kaffarah or can they just pay the kaffarah?
Answer 196: If it was not done deliberately and willingly there would be no obligatory on him to give Kaffarah. He must observe the Qada only. Because, commission of the things which invalidate fast are a cause of expiation in the same way as they make it obligatory to compensate for it, when it is done deliberately and willingly without any coercion.
According to Ayatollah Sistani (ha), it is obligatory on him to observe the Qada only.[1]
If an unbeliever becomes Muslim, it is not obligatory on him to offer qada for the fasts of the period during which he was an unbeliever. However, if a Muslim apostatises and becomes Muslim again, he must observe qadha for the fasts of the period during which he remained an apostate.[2]
For further information in this regards, please refer to the following answer:
Index: Things from which a person keeping fast must abstain & Kaffarah (Expiation), answer 372.
[1] . Tawzihul Masael of maraja, Vol. 1, Pg. 891, question 1572; Tahrirul Wasilah of Imam Khomeni (English version), Pg. 324; This question has been asked the Office of Sayyid Sistani (ha) by AskIslam.ir.
[2] . The official website of the office of Sayyid Sistani, Rules regarding the Qadha fasts.