Mewtwo v Star v Star Rainbow

My six year old son recently discovered Pokemon and is obsessed. My husband and I know nothing about the cards, the game or the universe. Recently my son got a mute mute mute to the star Griffin. If we let him keep this card, he will trade it from Lord knows what. What's the protocol here? Let him make his deals, protect him from older kids taking advantage,. sell the card now and buy him a bunch of other packs of cards with the proceeds that's from poke a perplexed and Pittsburgh.