Ok it still sounds like it could be some arthritis or tendonitis.
So to officially diagnose arthritis an x-ray would have to be taken. That would show the vet how the joint cartilage looks and if there is any arthritis. He is not that old so it could be tendonitis too.
Tendonitis is inflammation of the tendons in the joints.
Both arthritis and tendonitis can be worst when they first wake up and can improve with movement.
I also asked about tick exposure because Lyme disease can cause lameness and it can shift around to different legs.
So you could try over the counter aspirin for pain and inflammation. You can use it for 3-4 days and if there is not improvement, or he gets worse, then he needs to see a vet. Also aspirin is not a good choice for long term pain control because it has more side effects than some of the prescription veterinary pain medications.
So if he is 10 pounds he can take half of a 81mg aspirin tablet every 12 hours.
Give it with food to help prevent gastrointestinal upset.
In case it is tendonitis you should restrict his activity until he is no longer limping.
So if he is not better in 3-4 days then he should see a vet.
Does that help? Do you have any additional questions?