Looking on the internet deeply has found these results:

keep his promise is the most popular phrase on the web.

fulfill his promise 1355,000 results on the web	keep his promise 12,010,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
Garraf and I fulfill my promise. He's stealing to fulfill a promise. But our nation cannot be whole if Tutankhamun fulfills his promise to honor Aten as himself. And I always fulfill my promises. You will help... to fulfill its promise. Martin was a shy, awkward child who will never fulfill his promise to become a weird, unloved adult. Now fulfill your promise and let us go. I'm also here to fulfill a promise.	Shiro had to go to keep his promise to his dad. The banner of blood, kept in my family for 1,000 years, lest we needed Sant'lago to keep his promise. Nagtso became extremely anxious since now it appeared impossible for him to keep his promise to the Abbot of Vikramashila. Alberto is particularly moved: he managed to keep his promise. Tom failed to keep his promise. I believed that he would keep his promise. We expect him to keep his promise. If he's a man of honour, he'll keep his promise and give these men their raises.

fulfill his promise

1355,000 results on the web

keep his promise

12,010,000 results on the web

More popular!

Garraf and I fulfill my promise.
He's stealing to fulfill a promise.
But our nation cannot be whole if Tutankhamun fulfills his promise to honor Aten as himself.
And I always fulfill my promises.
You will help... to fulfill its promise.
Martin was a shy, awkward child who will never fulfill his promise to become a weird, unloved adult.
Now fulfill your promise and let us go.
I'm also here to fulfill a promise.

Shiro had to go to keep his promise to his dad.
The banner of blood, kept in my family for 1,000 years, lest we needed Sant'lago to keep his promise.
Nagtso became extremely anxious since now it appeared impossible for him to keep his promise to the Abbot of Vikramashila.
Alberto is particularly moved: he managed to keep his promise.
Tom failed to keep his promise.
I believed that he would keep his promise.
We expect him to keep his promise.
If he's a man of honour, he'll keep his promise and give these men their raises.