"Make sure"? Short answer? You can't. Once he has those files he can do anything he wants with them.
Longer answer - you could try a contractual agreement like an NDA (Non-Disclosure Agreement). But they are only as good as his honesty and your lawyer.
Another option he has - He could BUY an actual product from you and take it to the manufacturer for an estimate too. But since his manufacturer is most likely in China (Grrrrr!!!) that is a little more difficult than just emailing drawings.
Your drawings are your most valuable asset. Share them at your peril.