"To Tighten non-riveted joints, take them apart and dab a little bit of Future Floor Wax into the socket. Two or more coats might be required, depending on how loose the limb is. " - Jay

"Loose riveted joints can be tightened by CAREFULLY squeezing either end of the rivet with a leather hole punch or similar tool. EXTREME CAUTION MUST BE USED TO PREVENT CRACKING THE TOY!" - Jay

Adding a small drop of Hot Glue may allow for a temporary fix to this problem. The Hot Glue would need to be worked in the joint and it would toughen it up for a while. However, there is no permanent fix other then tightening up the rivet or replace the rivet entirely.