Brushing baby's teeth

[Rookie889]

I have a 9 month old son. He has 8 teeth already and we're trying to brush them with zero success. Any tips? Tried it with my finger, he bit me (and those teeth are sharp!!). Bought a baby toothbrush and he just chews on it...I'm worried as they're starting to go a bit yellow.

He does drink water from an open cup, never a sippy cup, and he is only breastfed, no bottles. I have been giving him blueberry porridge every morning which is veeeeery purple (they're frozen blueberries and they go squidgy when they defrost and make the porridge purple). Could that be the cause of his stained teeth?

He doesn't have any sugary snacks or baby food pouches, only homemade food (Dh and I cook a lot).

I should say I had a lot of teeth problems growing up as I had a fizzy pop addiction ages 7-10 (my parents didn't know any better at that time) so I am very worried about teeth generally.

Their teeth are so sharp!! I have two baby toothbrushes so she can chew one of them while I try and brush with the other. It's not perfect, but better than nothing. Where I can, I go over the top with showing the baby what I'm doing With someone else (e.g., pretend to brush my husband's teeth / her older sister's teeth). Sometimes that helps. With her older sister, I at some point (on the advice of a dentist!) caved and let her watch something like Hey Duggee Brush Your Teeth so I could brush her teeth and then weaned her off that eventually and now she's 4 and she just let's me brush her teeth. No idea about the porridge. If you're registered at a dentist, you might be able to register the baby and start taking them with you? My dentist didn't let me register the first one till she was almost 2, but they've been happy to register baby 2 and she is coming along to my next dentist appointment.