Go to the vet - if it is a dental issue, it is now beyond the point of being able to be managed by toothpaste or any dental water additive. Think of it this way, if you have a toothache so bad that you are unable to eat, brushing your teeth doesn’t solve the problem - you would go to a dentist. Brushing their teeth helps prevent tooth decay and other dental diseases; but it will do nothing to help once a problem has started.
a hedgehog not eating and struggling to eat is very very serious and needs to be addressed urgently. Anaesthesia has a very very low risk when your animal is otherwise fit. The longer your hedgehog goes struggling to eat, the more likely he is to lose body condition, and with that comes a higher risk with anaesthesia as you have a weaker animal. So go now, so if he does need to be knocked out for some dental work, he will have a very low risk and should be totally fine once his teeth are all fixed up.
once the issue has been addressed, then you can work on managing his dental hygiene to prevent more issues going forward. I personally prefer toothpaste over water additives as I find these to be more effective. I use virbac enzymatic toothpaste and a long handled cotton applicator to brush my guys teeth 2-3 times a week.
hedgehogs should also have a water bowl, not a bottle. Metal or no metal, water bottles do not dispenser enough water at a time and instead only release tiny droplets. A water bowl allows them to drink freely and unobstructed, and is overall much more hygienic.