Great partner

by not responding anymore.
Looks more like he cut you out of the loop and continued with that other partner.
Do you only have his email address?
Give him a call otherwise.
And if that don't work send him a signed letter, don't know exactly how to call it in English, a certified letter?
Anyhow, the mailman delivers the letter and only he can take the letter in receipt by signing in and identifying himself.
If he still doesn't respond then then you know exactly where you at.