If he's to remain intact? Never. Even the best of dogs can be tempted out of their own yard by a bitch in heat.
Leaving dogs in the yard unsupervised is how all manner of very heartbreaking posts on forums begin. My dog was attacked... my dog was poisoned... etc.
When my bitch is out of season, I watch her from the window when she goes out. I'm lucky that she has a particular spot that she uses as "her latrine", that is viewable from our kitchen window and back door. When she is in season, she is of course always on a long-line with myself or my girlfriend at the other end.
As others have said, for now, I would continue to accompany him so that you can lavish him with praise when he goes outside appropriately. In a couple of months when he's older, the weather is milder [working dogs or not, some dogs just don't like to be cold/wet, and will go outside, decided against it, and come right back in without having gone], and his recall is a bit more reliable, you can start letting him out and watching from the patio door.
I personally wouldn't ever get in the habit of not watching my dog in the yard- it only takes that one "he's never done that before" for a completely avoidable tragedy to occur, and if you are consistent from the beginning, you can have a dog that goes straight out and comes straight back in, making it no trouble at all to wait and watch from the door.