I would probably assume he is fine to return on Monday because likely he will be more or less symptom free by then and you won't even know that he was still a bit sick on the weekend and parents for sure won't tell you.

As to the other family getting it there is always the argument that it is out in the community and child was kept home the full week so if they now get sick then there is only a remote chance it came from the now sick child.

At the same time do not plan shared activities next week as much as possible - no cooking together, no playdoh, no sharing anything you can't wash although so what at the end of the day it is too late anyways.