I can vouch for Nate's suggestion with proto.io, great tool for making a fully clickable / functional prototype, and fairly quick to learn... something like flinto is even easier, but not as capable... take a screen recording / video of your clickable prototype and include it in your powerpoint presentations, it gets the idea across very well -- i had some trouble describing my idea before doing that, but it became abundantly clear to people when i did
I also second the notion that you don't need to worry much about idea theft. 1) Your developer will be far more likely to stay with you than to steal your idea, if he likes it 2) It will be other people that copy you, after you release, and you can't do much about it but stay ahead/execute better
Regarding holding onto your equity, read up on repurchase agreements and vesting. You can always offer equity, subject to repurchase by the company -- you'll just have to give them a good enough repurchase price and enough equity to incentivize them... it enables you to offer a decent chunk of equity without worrying about giving up "too much value" if your company does really well and the value jumps... (you could always have the company repurchase them if the value skyrockets -- though if your developer was a big reason for the success, you'd probably let him keep the equity as a "thanks" -- but at least the power is in your hands, which gives you comfort that you're not giving up too much if things go really well... and obviously if things don't go well, then you're essentially getting free work, which is why you have to offer a fair upside to compensate)
I'm also offering warrants to my devs, allowing them to purchase more shares at a discount -- if they put some cash in, that's more skin in the game which is much better for alignment... e.g. if you decide that your company's shares are worth $0.20 for the sake of valuing their hours, allow them to buy an additional x shares at $0.10, and y shares for $0.15. I also set up a cliff, so no options/shares are issued before 1000 hours of work are done, which they're more than happy with.
I can show you a spreadsheet I used to calculate all that stuff if you want -- I also shared it with them, so that it's totally transparent how I came up with "you earn x shares per hour of work"... just be honest with them -- you deserve a larger share because it's your idea and you're the only one who's put money in so far -- they'll understand that, especially if you give them the option to buy in for more. As for the legal agreements, you might set things up one of two ways (i'm using both)... a really good person who you're super comfortable with, and want as a "cofounder", you'd probably issue shares up front, at the same time you issue shares to yourself... these shares are subject to repurchase (ultimately a vesting schedule)... the second way of setting things up, more for "contractors" you might use options, with a cliff at 500 or 1000 hours, to make sure they get you something useful before you go to the effort of formalizing anything on their behalf -- it commits them psychologically as well. Keep in mind that essentially everybody is a contractor to the company legally, even you... so you will all sign contractor agreements, and should define duties therein
Happy to shoot the shit more, been a while!