Good morning, Christian.
It's sounds like Charlie's neck or back may be hurting him. This is a pretty common complaint in dogs, in fact.
Small breed dogs are prone to a condition known as IVDD or intervertebral disk disease and also to neck and back spasms. This commonly manifests itself as discomfort in the neck and back. Many dogs will avoid taking stairs, jumping onto or off of elevated surfaces, etc. 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/ and here: http://www.petmd.com/dog/conditions/neurological/c_dg_intervertebral_disc_disease
He needs to be kept in a kennel or other small area with restricted access in movement (bed rest, essentially) until your vet can examine him. This should be padded well with plush blankets to help support the neck and back. He can continue go outside to potty but should be carried and placed gently in the grass and then carried back to his kennel. Do not encourage or permit playtime, running or excited activity like jumping even if he begins to feel like it has it may worsen the symptoms.
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 medication if it's indicated. Strict kennel rest will continue for a few days to a few weeks based on his condition. If IVDD is diagnosed, a referral to a specialist may also be recommended.
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