What a horrible situation to be in, it does sound like your partner is depressed as well as you, but I'm afraid he needs a wake-up call as well as getting some help regarding his current state of mind. My partner dislikes his job and cannot find anything else at the moment he would rather do, but he knows he has to stick with the job for the meantime as we have a young child and he has debts, otherwise we wouldn't be able to afford our rent, bills, childcare etc.
Your partner is obviously a bit more than a daydreamer, he is clearly deluded. Which is fine if you are single with no-one else to think about, but he must be mad if he thinks that writing a novel is going to feed and clothe your son. Yes, there is a small chance you have the next Jonathan Franzen tapping away in your spare room, but even he took up to a decade to write each of his novels.
As other posters have said, I think he needs to accept he is depressed, and that he is being unfair on his child, and agree to take steps to get some help through his GP with this. I think he also needs to accept that he is in real danger of jeopradising your relationship, so you do need to threaten to leave if he doesn't sort himself out.
With your job, you may find you don't lose much in the way of benefits if you declare the extra work, all depends on how many hours you are working and how much you are taking home.