In my experience, a request like that would break the mood and change the dynamic: If he slips his dick into your willing mouth during the massage, then he's controlling how erotic it gets. If you get up from the table and climb between his legs and start sucking him, it becomes an escort session for which he deserves a higher fee. Alternatively, you could ask the masseur in advance for a rate that includes sucking his dick. (Or hire an escort.)
Others might disagree with me, but when I choose a sensual or erotic masseur, I try to choose carefully and then just let them do their thing.