Glad to hear he's doing well. How long has it been since the surgery? All dogs are so different in how they recover. Some take two weeks, others take a month. Laying on cold ground is soothing for swollen, painful areas. It sounds like he might be in pain. What pain meds is he on?
The biggest obstacle that dogs face after amputation surgery is weight. If your dog is at all overweight, he will have a harder time getting around.
Also, it takes time to build up muscle strength in the remaining limbs. He needs to learn to use those muscles differently now. You can help him along with Tripawd stretches and exercises that you can find on our site, Tripawds.com.
Please come visit us there at Tripawds, you'll find lots of great advice and support from others who have gone through recovery. Good luck to you and Ranger!