Guinea Pigs change in behavior

Question
Hello! I hope you can help me understand our new guinea pig.  We got him (Jake) a month ago, and he is an adult (not exactly sure how old he is, but not more than a year or two). When he was first in our home, he would run (almost frantically) around the room and try to hide- I figured that he was a little nervous in his new surroundings and was exploring.  About a week ago, he started to relax, but now he seems to have mellowed out too much.  Now if I put him in the middle of the floor, he will just sit there. If we put him outside in fresh grass, he will just sit in one spot and not explore at all (and eat only the grass within reach).  He is eating in his cage without any problems (good pellets, fresh greens and veggies twice a day, hay available to him- although not his favorite to munch on), fresh water daily, a nice "house" to crawl into and on top of which is made of wood that he likes to chew, and various toys that we will put on the floor for him.  He has also started to "drag" his back feet when he moves.  I was concerned that something happened to his feet, but I can touch and move them without Jake showing any signs of being in pain; and he does use his back feet when he has to.  But, he will "scoot" a few feet, usually dragging his back feet, and moving slowly.  He does not seem lethargic; he coos and squeaks softly when we pet him, and I sometimes hear him chattering to himself or my daughter.  He does shed his fur, but it does not seem to be an abnormal amount (he has shorter hair), and we brush him with a soft brush regularly and give him a gentile bath (which he seems to enjoy).  I have not noticed any diarrhea, which I know can be a sign of stress.  Although he moves freely around his cage, he has virtually stopped running around and exercising.  Perhaps we have the laziest guinea pig... or maybe there is something wrong with him.  I will appreciate any suggestions or insights! We really like having little Jake, and my daughter adores him! Thank you for your time!!!

Answer
Hello,
I think that before suggesting to take him to the vets I would try to get him to be a bit more playful. Try putting tubes in his hutch so he can run around through them. You could also play with him more so then he can get used to the sound of your voice. You could buy him a lead so that you can watch his movements as he runs around. I definitely think that you should take your guinea to the vet because although it doesn't look or feel like he has something wrong with his back legs he may need an x-ray as a bone may have dislocated or broken and it may hurt him to move so he doesn't move apart from to eat. Although this is serious it may mean that your guinea will also become fat and lazy so when he gets better he may not be as playful.
Best wishes and I hope Jake gets better.
Aleksandra