Ben, there’s some info above ranging from barely accurate to wildly inaccurate, particularly given that no one knows of the terms of your original arrangement or settlement, or indeed the plans for your business that may involve aspects of that design.
You are not obliged to sign the NDA, that much is correct, but I’d suggest you seek proper advice in respect of any other aspect of this (I know you’re not daft though, just felt a line had to be drawn under what is correct and what is Gordon Gecko strategies based on guesswork and assumptions) 🙂
Any interaction (or even non interaction) with the designer could have an impact on your future relationship and legal position, so this needs to be taken into consideration before you do anything.
If he is trying to firm up his position re IP and perhaps has other people with a financial interest involved it’s probably worth exercising a bit more caution than normal to avoid doing or saying something that may prejudice your position.
Also, a wise man once told me “Never write anything in an email that you wouldn’t be happy hearing read out in court”. 😉