5 month old baby drinking lesser and lesser milk

Hi mummies, My baby is coming to 6 months old. Only started to introduce him brown rice cereal yesterday. However, I noticed his milk intake is getting lesser and lesser. Today he is drinking less than 100 ml per feed every 3 hours. Very worried that he is not going to drink enough milk. Is it because he is teething (though no teeth yet) or coz he was introduced to solid?