I have a corgi/jack russell?

There are much better and safer medications that ibuprofen to treat your dog's arthritis. It sounds like he is really hurting at times so I really encourage you to follow up with your own vet, or maybe even a different one if your current vet doesn't seem willing to take your dog's discomfort seriously. Other treatment options to consider include: carprofen (e.g., Rimadyl, tramadol, gabapentin, amantadine, amantadine, acupuncture, cold laser therapy, nutritional supplements containing glucosamine... the list goes on).