Why start by writing him? Emails are easy to ignore. Give him a friendly call. That'll kick it up a notch without seeming hostile. You don't need to act like

Why start by writing him? Emails are easy to ignore. Give him a friendly call. That'll kick it up a notch without seeming hostile. You don't need to act like anything is amiss -- just say you're following up on the invoice you sent on April 30th. End the conversation with "Great, I'll look forward to receiving payment by June xxth. Thanks for the project, and if there's any more work I can do in the future, just let me know." This (1) sets a firm date (even if his answer is "ah, well in the next week or two...") and (2) keeps it friendly. Personally, I wouldn't acknowledge that this is a significant financial issue for you. It doesn't matter. You're entitled to on-time payment whether you have one dollar or ten million.