Most Helpful Guy
I'd suggest being direct in the moment. Talking with him about what you think you can handle before hand should also ease his mind. If you're not sure make something up that you think you would and go on the gental side so you don't scare him or make him think you don't know what you are talking about (even if you do). For extra piece of mind use a soft safeword or something and use it a couple of times but tell him you pulled the cord early so he gets used to doing it and knows you would stop him if he went overboard.
Example on 1 to 10 of X thing you think you want 6 and can take 7 if he goes to hard. Tell him you want 2 and you can take 3 and if he hits 2 stop him then say that you think you could go to 4 an repeat until you are both comfortable with whatever.