I have a senior boxer who in this rainy weather is having a very hard time with his arthritis. Can I give him tylenol and if I can how much. He weighs between 75 and 80 pounds.

Talk to your vet before giving him anything. My German shephard is ten and a half with hip dsyplaysia and arthritis . He can still get around easily, but jumping up in the jeep or on the bed some days are harder. I have been giving him Deramaxx,for almost a year now. He gets a half pill every 5 to six days. He can have a half pill each day if needed for pain.
Ask your vet.

I would also discuss with your vet, a special diet that includes glucosamine and condroitin, along with calcium, and other supplements such as Vitamin D, fish oil, flax seed oil, etc. that will help with bone deterioration.

Always ask your vet before you give your dog any pain killers. I don't recommend giving him a tylenol. Even if you did, you don't know the dosage. Some people will give their dogs an ibuprofen, but I have heard some bad stories about that too.

Just check every medication with your vet first. Also, ask the vet ways you can massage your dog to release the tension and soreness of his arthritis. I don't know if this an available service where you live, but I have heard of some success with acupuncture for your dogs arthitis. Just a thought. I know if my dog was hurting like that, I would do anything to relieve her pain.