He is wearing a king's crown.
Explanation:
A painter interpreted the story of Ramayana in the "Illustration of Sugriva Challenging Vali from the Ramayana. Sugriva roars from the outside of the place of Kiskindha calling his brother Bali. Due to this, many birds started to fly because of the sound. Rama and Lakshmana were standing and looking these from the right side with some monkeys along with them.
With the response to the Sugriva's roaring, Bali and Tara, Bali's queen, came out of the palace of the monkeys king which is illustrated on the right side. It was illustrated that Bali and his associates were wearing dresses that is normal as human wears and instead of turban Bali was illustrated to wearing crown.