If that's your proposition you'll need to provide your web developer which an idea of how many days/weeks/months of work you'd like from them.
You'll also need to start with a cash flow projection and projected P&L accounts so that the designer can see what you expect to be turning over and making as profit.
Then you'll need to provide an indication as to what the 'small equity stake' will be worth in say 5 years time, if for instance, you're paying a dividend of say £50K and the developer is getting 10% of that (assuming you've not raised more cash and diluted his shareholding, or paid everything out as salary and bonus to leave no dividend to the share holders) then a £5K a year dividend may give the shareholding some value.
Lastly you'll need to provide the designer with an exit route. Now that he's got his % stake in the business, how will you ensure that he can sell his shares, and who to, and at what value? Are you going to personally guarantee to buy the shares back from him for a minimum value at some point in time?