If he is living abroad for more than 6 months at a time, He is not entitled to receive OAS or GIS except CPP. However, one needs to inform their absence to service canada about long absence. If not informed and he continues to receive benifits, service canada will come to know. There will be fine for not informing as well as benifits will be stopped. Then again need to reapply once he comes back to canada.
The thing is one needs to be honest...yeah.