My dog stopped jumping. What's wrong with his hips, back, or joints?

I am very sorry to hear that Jake seems less eager to jump then he used to be. This is a very common problem with older small- medium dogs. Though it is difficulty to say exactly what is the cause, I would be worried about osteoarthritis in Jake's hip joints. A vet would be able to diagnose with a physical examination as well as potentially using an X-ray to visualise the changes in Jake's lower back, Such as slipped discs, knees, and hip joints.

Because Jake seems otherwise very well in himself I would advise adding a supplement to his diet to help with joint health. There are several of these on the market currently. Look for Glucosamine & Chondroitin in the labels, and always buy a pet formulated brand, as doses are different for humans.

I would also advise a change in his exercise regime to promote lean muscle mass development. Muscle wastage is a common contributing factor in older dogs. More frequent, but shorter walks will benefit Jake. For example, two 20 minute walks, One in the morning and one in the evening

If you notice that Jake is particularly sore after a walk you can apply a cool compress to hip area

I hope this answer was helpful and if you believe that Jake is not benefiting from these tactics I advise a trip to the veterinarian to ensure noting more is occuing.