I would say bike with him two or three times, if you can. Bike instead of drive that first day if that’s what it takes to be able to do this more than once.
The reason I am urging this is twofold: first, so you can get a good feel for how safe (or problematic) the route is at the time of day he’s going to be riding it. (And I don’t think once is enough: you can get fluky days in terms of traffic, or simply find it easier to ignore whatever problems you do come across because you’ve only experienced them once.) And second, so you can get a good read on how he is reacting to the road crossings, whether he feels nervous or threatened by them, or whether he’s taking them in stride.
Once you do this ride with him a few times, and see it as he’ll experience, and see how he reacts to it, you’ll know how comfortable you are with letting him ride this route, and you’ll be prepared to make your own call on whether he can handle this himself, or whether you need to keep on chauffeuring him.