My son hates when I try to clean his tongue. He has developed a bit of yellowish colour on his teeth. We brush daily once.

Kids do hesitate to clean the tongue. Make it a fun routine. Sing rhymes, tell food stories on how to keep the teeth clean. Emphasis Oral hygiene is very important but don't push. Show videos on you tube on how other kids do it. Brushing twice daily is fine.
Top 5 Myths About Infant Oral Care Busted!

Hi there are personalised brush, towel, even paste is available. Buy that and make brushing time fun.