Gracie was 3 when I got her and already had pretty bad damage to her teeth. She had teeth cleaning by a vet a couple months before I got her and I wasn't brushing either hers or Lady's teeth often. I recently brought both girls into my vet, she said Gracie had a few pulled teeth, some bad chips and already some pretty heavy build up. This was only about 6-8 months after her teeth cleaning, and I use a water supplement called Healthy Mouth. I stopped giving bones because of it. Like you, it made me feel really bad!!
I started brushing their teeth every single night a few weeks ago. The flavoured toothpaste really helps!! They both think it's like a nightly treat now and come get their teeth brushed no problem. I bought them each their own kids toothbrush with the big grippy handles so it's easy for me to hold and it's small enough for their mouth. With Lady it's more of a challenge, she doesn't really want me in there but she likes the chicken toothpaste it's just getting her used to the brushing. Even with brushing every day they still have build up on their back molars so I bought a gel called Tropiclean and I'm hoping that combining brushing with the gel will help.
Maybe you could start slowly by introducing him to a toothbrush with a little bit of the toothpaste, and gradually work your way into his mouth and he may get used to it. Just be patient and make it an enjoyable experience. I know I can get frustrated with Lady sometimes but I think it will be worth it!
If he really really really wont let you in his mouth to brush then I think gel and a water supplement will be your best bet and have annual teeth cleanings at the vet.
Lady (1.5 year old tricolour) & Gracie (4 year old blenheim)
"Happiness is a warm puppy" - Charles M. Schulz