Focus on your breathing. If you're nervous, you're likely to breathe more slowly and shallowly - if you can consciously breathe more deeply and slowly, it will help convince your body you are calmer. Do this while you're waiting to go in, rather than concentrating on it solely through the interview, so you don't hear the questions properly... But a quick thought from time to time throughout the interview can help.
Basically, it's fake it till you make it. Shake hands firmly. Be aware of your body language - try and sit openly, rather than hunched over and head down. If you're feeling shaky, put your hands in front of you on the table, with interlocking fingers (or on your knees, if there's no table) - that should help stabilise you (and maybe best not to wear potentially clanky bangles, if you were thinking of it.) Look them in the eye.
Don't forget you don't have to speak immediately. It's okay to pause for a moment - thinking time - take a breath, and then start answering. We've a tendency to speak faster when nervous, so try to speak more slowly. And if you get to the end, and there's silence, don't feel tempted to fill the space with waffle. If it's getting uncomfortable, you could ask, "Is there any part of that you'd like me to go into in more detail?" or something like that.
Be as prepared as you can be - if there's anyone you can ask for more detail about the role, and then think about how the things you currently do - and also experience from past jobs, voluntary roles or wherever - can be used to fit into the tasks you'd have to do in the new role, so you've got a mental list of examples to hand.
I know it's a lot to think about when you also have to think about the actual interview questions, but if you know he's a nice bloke and you already get on well, then you've already got something on your side - and they wouldn't be interviewing you if they didn't think you had a chance.
Good luck!