Hello, I'm avail. at the moment and can try to help you.
Paracetamol is acetaminophen. Dogs do not metabolize acetaminophen as well as humans so if used, it must be done carefully and at the correct dosage for the correct period of time. For this reason, many vets don't use. acetaminophen for dogs at all. There are so many much better and safer drugs available (mostly NSAIDS) that are approved for use in dogs.
For a large dog the size of yours, however, the toxic dose is way more than one 500 mg tablet. 1 kg = 2.2 #, so your dog weighs approx. 50 Kg. 100 mg/kg can cause liver issues, over 200 mg/kg can cause methemoglobinemia. However, there have been reported problems at lower doses too. Your dog should be okay at the dose he got, but to be safe, you may want to talk to your vet about using an dog-approved NSAID instead.
Also, there are a lot of joint nutraceuticals out there that are safe and may help your dog long-term, usually chondroitin and glucosamine combos:
Best wishes for your pet!