At 17 months you need to still be brushing his teeth not letting him do it. My dd always resisted teeth brushing too but we made it non negotiable even it it meant tears every time. It took a lot of months of fraught teeth brushing sessions but she grew out of it and at 3yo brushes her teeth beautifully (with a bit of help). Dentist wise I took dd with me to my annual appt at 18 months or so to acclimatise her to sitting in the chair opening her mouth etc. then she had 6 monthly appts from 2ish
I was in this situation and I had a couple of evenings where I forced her to let me brush them. Waited til she was in pjs after a happy fun bath then lay her down on our bed and pinned her down and said let mummy brush them then we can have stories and cuddles. She hated it the first few times but now lets me do it with little struggle, and almost quite enjoys it. I go 'bzzzzzzzz' as if it's an electric toothbrush. Only in the evenings mind you, I let her do her own in the AM.