The documentation of the swim test would have been issued by whomever conducted the swim test. I would suggest your scout reach out to his Scoutmaster for any documentation if he doesn’t have it. That said, swim tests only last for one year, so it sounds like he is due for a retest soon.
The only place that I know of to record the Swim Test dates is in the scout’s profile (My Dashboard -> My Family -> Scout’s name -> Edit Profile).
You might be able to put photos of any documentation you find in the comments for Second Class 5b or First Class requirement 6a. It’s a convenient spot, since each is related to demonstration swimming skills.