I got this wrong for a simple mistake. In my day-to-day life most of my writing or typing is very weird and formal sounding because I'm dealing with certain types of clients. So I use phrases along the lines of "it belongs to them" or "it falls under their area" instead of "it's John's" or "it's one of theirs". So I got this wrong because I types "sons" instead of "son's". I just haven't had to physical type or write the possessive 's or s' for a long time. Weird how taking a course on Swedish is helping me relearn my own native language.
You would normally say "den" about a dog, but many people who consider their pets family members might use "han" eller "hon".
But note that the "det" in this sentence is not the dog itself, but the formal subject referred to by Arnauti in this thread, and expanded on here: https://www.duolingo.com/comment/9708920.
There's subject and then there's formal subject. "min sons hund" is the subject and "det" is formal subject.
Explained at bottom here.
And here. <- This is a google books preview, if the link doesn't work for you google "det formal subject" and click on the google books linking to "Swedish: A Comprehensive Grammar"