Tell him what you found and you’re not happy. Let him offer recompense. Depending on his response, then leave feedback.
I would have done a bit of ‘tyrekicking’ before completing the sale though. If the police had stopped you, it’d be you that had got done. It’s the driver’s responsibility to check the road worthiness of the vehicle. I wouldn’t have driven for 2 months without checking the tyres. I check weekly, especially with the poor state of the roads these days.