It will be fine.
Jax has been under 3x now - once for neuter, once for dental surgery and cleaning, and then once just for cleaning. He really hated me after the last dental cleaning. He was so out of it for 24 hrs, did not want me near him, wouldn't come near me. He got over it within 48 hrs though and was back to himself.
And it's really worth it... staying on top of dental care is so important. And if I were you, I'd rather do it now at 8 while he's still relatively young and then keep up with them with brushing every night and hopefully not have to deal with many more dental cleanings in the future.