No. If it was son's, you would only have one son and Matej is his friend. But you have more then one, so you have sons. And your SONS' friend is Matej.
Hello DL, I had the same problem. The only available tile is sons. Could you add sons' or just ' ?
There is definitely an apostrophe missing - it should be either "son's" or "sons'. " - so, with an apostrophe either before the s or after it. My Czech isnt good enough to say which of the two apostrophes are right.
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.
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).