He is the friend of our sons, in the plural, not singular. Or, he is our sons' friend (with the apostrophe after the "s" to indicate possession in the plural - possession by more than one son. For some reason many people are confused about apostrophes work, so I write it for others).
If you look above, you will see it is indeed sons'. Perhaps it is missing in the word bank, but we have no way to influence that. I recommend using the website to write your sentences using your keyboard instead of clicking on words in the word bank. Even in the mobile phone I like the web version more.
Well, what specifically did you look up and found confusing? You do not want us to repeat everything in the same way to confuse you again, do you? At this stage I can only give you links to Wikipedia or other resources, which you probably found already https://en.wikipedia.org/wiki/Genitive_case