For him to have chosen to make use of a particular gambling site not minding the interesting bonus offers from other gambling sites he would have been enjoying some services that he does not find in those other sites you are bringing to his notice. And like myself I don't give so much relevance to bonuses as what I am very much interested in from a casino is that fairness and how fast their support system is when as a gambler I face certain challenge with my account.
Op I think you stop trying to convince your friend about it and let him enjoy what he's enjoying while you enjoy yours. We all have out different interest.