How do you force Google Search App to keep Safe Search Filtering on
After quite a bit of searching and testing I don't think it's possible to do. Even locking SafeSearch on your Google account on the PC doesn't change it for the phone. 

Regardless of this I would imagine that if your son can turn SafeSearch off even if you did lock it he would probably try getting around K9. This isn't too hard, there's even a suggested method in the product reviews, it apparently doesn't stop you from browsing within another app.