You are confusing the term "unlocked". The D3 on Verizon is not locked to Verizon's CDMA 3G network. The phone is shipped from the factory to be used on Verizon's network. The GSM portion of the phone is locked, and is not programmed for use on the North American GSM carriers.
If your brother stops paying for his contract, and doesn't pay his required Early Termination Fee, the phone will be blacklisted for use on Verizon's network. The phone will become known as a "Bad ESN" or "Bad MEID" phone. Page Plus utilizes Verizon's service, so any phone blacklisted by Verizon CAN NOT be used on PagePlus.
There are other CDMA services that the phone can be used on (Cricket, Metro PCS for example), even when blacklisted by Verizon. The "Bad ESN" phone would need to be "flashed" (programmed) for use on Cricket or Metro PCS. You would have to pay someone to flash the phone.