The plastic is really thin at that point as it is where his knee is and is also the pivot point for the movement. The force of going back and forth just does it in over time.
I tried several things with no luck due to it being to thick as that part goes behind his stag-net leg and the back plastic. If you go too thick his leg gets stuck in the air and doesn't come back to it's resting position.
Only thing I didn't try would be to put some sort of metal bracket on the side. It would have to be really thin and then mounting it would be a challenge.
In a perfect world, a REALLY thin piece of metal that you double sided taped to the back of his leg would be the best option. Not sure if metal that thin is possible though but it would probably need to be about half the thickness of a cliffy.
Probably the best option would be to have someone make replacement legs in PETG and just apply a decal to it as a replacement. Stern wont make just that plastic so you have to buy the full kit for it thus why those that have a broken leg generally just deal with it. If I had a laser cutter I'd consider making that piece but I don't and making it by hand would be hard, if not impossible.