Some kids just take longer learning how to sleep through the night. I would strongly advise you to avoid looking for a medical problem especially if he is healthy in every other way. My daughter who is now almost twelve and still needs me to tuck her in every night, benefited from nightly arm and leg rubs with a nice soothing oil before bed every night. After a warm bath, dim the lights in his room, talk in a quiet, calm voice as you dress him for bed. Then sing to him quietly as you rub his feet and legs (and arms and hands if you want. Get his bedtime routine the same. As always consistency is the most important thing.
Are you a working Mom? - maybe he just wants more time with you. Do you nurse? My daughter didn't sleep well until she stopped nursing around age 2 1/2. What happens when he wakes up at night?
I really doubt that he has Restless Leg Syndrome - Dr.s seem to be so eager to give us a pill when they don't know what else to do. Patient beware.
Eventually kids learn to sleep through the night, just like they learn to use the potty and talk. I know it's rough when you are not getting to sleep all night but in the end this time of toddlerhood does not last that long. Soon he will be a teenager that you can't get out of bed!! Hang in there and don't worry - that's my advice.
D. K