Why does my dog cry out when jumping for treats? Is it a leg injury?

Hello and thank you for using Petco Pet Education Center, formerly Petcoach! Charlie sounds like he is having an issue that comes and goes. The most common causes of lameness in his back legs at his age are a ligament tear, knee cap luxation (moves out of its normal groove), followed by a foreign object stuck in his paw. It is also possible that he simply strained a hip muscle.
No matter the case, he needs to be evaluated by a vet to determine exactly which. He may just need a little rest and anti-inflammatory therapy, but a sedated exam and radiographs may be needed to rule out significant disease that could need surgery to fix.

Do not give anything over the counter for his discomfort as those medications can be quite harmful to Charlie. Keep him from running around and playing too hard until you can get him in to be evaluated by a veterinarian. While I'd recommend having him seen as soon as you can, his condition should be okay to wait for a day or two if you can't get into your vet right away. However, if his pain is unbearable, you may want to take him to an emergency vet tonight.

Hope this helps, and I hope Charlie gets to feeling better soon!