It is not fully known whether he's a Sikh or not it is surely known he's Sindhi-Punjabi but most of rumors and social media and even wikipedia states that his paternal grandfather named as Sunder Singh Bhavnani was a turbaned Sikh with his picture even displayed in turban on social media. Bhavnani is a common Sindhi surname but even Sindhi Hindus have extreme love and respect for Guru Nanak Ji and have picture of Guru Nanak Ji along with other deities. Now Sindhi can be Hindu or Sikh too depending on ethnicity and religion.
However his paternal grandmother was known to be a famous Punjabi Christian actress named as Chand Burke who has worked in plenty of Punjabi and Hindi movies
☝️Grandmother of Ranveer Singh named as Chand Burke was a Punjabi Christian actress who also worked in some Hindi movies and it is a confirmed source. By relationship Ranveer Singh also is a distant cousin to Sonam Kapoor. It is also written on Wikipedia that Chand Burke after first marriage to Niranjan got divorced and did love marriage to rich successful businessman Sundar Singh Bhavnani. Sunder Singh Bhavnani and actress Chand Burke have one son named Jagjeet Singh Bhavnani and daughter who they named as Tonya
Father of Ranveer Singh named Jagjeet Singh Bhavnani is also a successful businessman reported to have huge net worth.
But it is not fully known that his paternal grandfather was a Sikh or not but I have found this picture below on Google and even on YouTube which shows that he has Sikh roots but however individual beliefs could vary as person could be more spiritual rather than religious or believing in single religion or maybe multi religious as Ranveer’s grandmother was a Punjabi Christian
☝️Picture doing rounds on social media of Ranveer Singh's paternal grandfather on Facebook,Google,Insta and even YouTube which might or might not be true.
List of Hindi film families - Wikipedia
☝️He has Visited Shri Harmandir sahib plenty of times.