Dear Mr. Robert.
Well, since your prompt suggests you that he's a friend of yours, it is better you follow the style for an informal letter. Here are some notes on formal and informal letters and hope that would be helpful for you for this task;
different opening and closing. The chart below will help you remember this:Style Characteristics Opening EndingFormal
To someone you have not met, whose name you don't know Dear Sir / Madam Yours faithfullySemi-formal
To someone you may or may not have met, whose last name you know & use Dear Mr Brown Yours sincerelyInformal
To someone you know well, whose first name you know and use Dear John Best regards/ Warm wishes