It depends on the level of familiarity and the cultural context. If Robert is a close friend or family member, it would be appropriate to use "tu". However, if there is a professional or formal relationship, it would be more appropriate to use "vous".
Which one should I use then? He is not my friend, he is just male.
In that case, it would be more appropriate to use "vous" when addressing Robert. When in doubt, it is better to err on the side of formality and use the polite form of address until you have established a closer relationship.
