I had a full leg cast on when I was 6, so I wasn't a toddler and could understand my parents when they told me that I just need to walk on it and not baby it again. But I also remember that being easier said than done. My left leg was skinnier and paler than my right leg for almost a month after I got the cast off. It would tingle when I tried to walk on it and sometimes, if I tried to walk for an extended period of time, it would actually hurt. When a limb is immobilized for any period of time, the muscles can actually "shrink". Since your son doesn't have the right vocabulary to tell you these things, you need to remember that it is a totally different sensation walking after the cast is removed than when it was on. Make sure that you don't let him get in the habit of crawling. Make him walk on it, but take it easy at first. Have him walk around the house when you are home, but put him in the stroller or shopping cart if you are going to be out for long periods of time. Eventually the injured leg will regain the strength that he needs to be able to put pressure on it full time. It could take just a couple weeks, or it could be a few months. Remember, there is no magic time frame, it all depends on your son and you, and how much time and effort you are both willing to put into his rehabilitation. Toddlers are very active people in general, so it might frustrate him for a while to not be able to do the things that he is used to doing or sees other kids doing. Take him to the mall playplace or somewhere that there are other kids that he has to keep up with, that might help him along.
Best of luck! :)