Thank you for your question. I am sorry to hear your spaniel is not quite himself. The colder months can certainly affect our pets just like they can affect us! Animals don’t always show obvious signs of pain and sometimes we have to look for any subtle changes which may indicate a problem. From what you have described, it does seem like he is painful and something is stopping him from jumping up like he normally would. Depending on the source of the pain, quite often animals will continue with most of their normal behaviour and just become reluctant to do certain things ie jump up or down onto something. I would advise a trip to your veterinarian who can observe how he moves and carefully examine him. They will be able to palpate the joints of the limbs and also look for any signs of neck or spinal pain. They may decide further investigation is required such as imaging (ie. radiographs). The treatment plan will ultimately be determined by the diagnosis reached.
I hope your boy is feeling better soon.