Hello @clixmoney.
Yes, You should start brushing his 5 teeth and the gum area (where teeth are yet to erupt) as well.
Select a good Baby Brush for teeth and gently do it once daily.For Gums, you can use either Wet Gauge piece or a soft wet clean cotton cloth wrapped around your forefinger.
This will help him get rid of Milk deposits in his mouth.
For any more questions, please let know.
Thanks