Veterinary questions? Ask a Vet for Answers ASAP
Hi, Welcome to Just Answer. I'm Chris, a small animal Vet based in the UK. I am reviewing your question and will get back to you as soon as possible
Poor thing, sorry to hear this! Is he currently on any medication for his arthritis?
OK. It sounds like he is having an acute flare up of his arthritis caused by the excessive running. This may settle to some degree over the next 24hrs, but I would be concerned if he cannot stand at all.
I would suggest getting him checked over by your vet this evening if at all possible, as additional pain relief may be able to be provided. For example a non steroidal anti inflammatory (so long as there is no contra indication for this).
I hope that help? Please let me know if you need any further help or advice.