As you may well know, small breed dogs are prone to a condition known as IVDD or intervertebral disk disease. This commonly manifests itself as discomfort in the neck and back. We can see symptoms as minor as tenderness and as major as full paralysis of the back legs. You can learn more here: http://sevneurology.com/articles/intervertebral-disk-disease-ivdd-in-dogs/ This is often the cause when dogs spontaneously begin acting painful and are unwilling to be handled by their owners. I would highly recommend an examination and treatment for Flap Jack at your local ER overnight. If this cannot happen, a visit to your vet tomorrow morning would be ideal.
Your vet will likely recommend x-rays to check his neck and back and will place him on a canine-safe NSAID and muscle relaxers, as well as pain meds like tramadol. Strict kennel rest will be recommended for a few days to a few weeks based on his condition and if it worsens they may recommend more diagnostics or referral to a specialist.
If you have to wait, the best approach is going to be strict kennel rest with no risk of jumping or falling. He can be taken outside to potty but should be lifted gently to go outside and placed gently into the grass. No playtime. A harness will be more ideal over a collar and leash, at this point, as well.
If he is not on any pain medication, you can give aspirin in a pinch. Please see the following link for info about dosing and risks: http://www.veterinaryplace.com/dog-medicine/aspirin-for-dogs/
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