Dr.Debbie has given some great advice on checking nerve function in your boy. One of the other things she mentioned was a blood clot (also called a thrombus). I just wanted to give you a bit more information about this.
Most often these blood clots are caused by underlying heart disease. I don't know if your boy has a heart murmur, but even without an audible murmur your boy could still have heart disease, but so mild there was no murmur. Let me explain....
When I hear a murmur while auscultating the heart, it tells me that there is turbulent blood flow within the heart. Think of a stream - if it is flowing smoothly, you can hardly hear it. But if there are lots of rocks and a waterfall, there is turbulence and now you can hear it.
The same is true of the heart. When there is turbulent blood flow, a blood clot or thromboembolus may form. Your boy may have a very soft murmur that could not be heard, but was still causing clots. This blood clot can leave the heart in the large blood vessels exiting the heart, but then get stuck in many of the smaller blood vessels leaving those major arteries. When this happens, it can cut off blood supply to a limb. Suddenly the cat limps on that limb and is very painful.
Think of wrapping an elastic band tightly around your finger. Pretty quickly it becomes painful and it is hard to use the finger. This is essentially the same thing that happens when a thrombus blocks blood supply to a cat's limb.
On exam, these kitties present with pain and limping. If the clot has gone to the right front leg (the first "exit" from the major arteries) that foot will be colder than the others and the nail beds will be blue. The blood clot may lodge in the caudal vena cava, which is a major blood vessel leading to both back legs. If it has completely blocked ALL of the blood to the back legs and tail, then the cat will not be able to use the back legs but will drag them around and will be in severe pain. Also, the pads will be notably cool, the nail beds will be blue and the cat will not have pulses on the back legs. This carries a grave prognosis. If your boy had a smaller clot, it may have just temporarily affected blood supply to the back legs, and then moved on to the front leg. My concern is whether there is still a blood clot floating around in his blood stream!
Early treatment involves drugs to stop more clots from forming - the choices are aspirin or coumadin (rodenticide, rat poison, warfarin). Unfortunately, if there is a major clot to both back legs, treatment may not be possible.
Here is more about thrombus formation in cats:
And more about Hypertrophic Cardiomyopathy which is the most common heart problem in cats, and the most common reason they develop blood clots:
If this is a blood clot, then early treatment is absolutely necessary before the clots gets lodged somewhere permanently. Thus, I would strongly recommend that you contact a vet IMMEDIATELY and take your kitty in right away.
The above is given for information only. Although I am a licensed veterinarian, I cannot legally prescribe medicines or diagnose your pet's condition without performing a physical exam. If you have concerns about your pet I would strongly advise contacting your regular veterinarian.