Hello and welcome to Just Answer. I am a licensed veterinarian and would be happy to answer your questions.
Zyflamend has been used safely in dogs, and has shown some promise as far as pain relief. Keep in mind that the glucosamine works via a different mechanism and is not so much for direct pain relief, but more for joint support. The two could actually be used together.
As far as doses, since it is not a drug, but a supplement and no specific studies have been done on dogs as far as it's use, all doses are extrapolated from the human dosing recommendations. I would start with 1 daily and see how he tolerates it.
Hope that helps. Please let me know if you have any further questions.