My DDs have walked the 20 minutes to and from school since the start of year 5, so aged 9, with no problems.
I second what @doublesheesh has suggested - we all have the Life360 app on our phones (it's free to download and very easy to navigate). It means you can see exactly where the other person is in real time, even zooming in so you have a 'street view', so you can tell exactly when he has crossed the road that you're worried about (and can relax a little!) We have me, DH, both DDs (aged 10 and 12) and my DM on our Life360 and have marked several places on the map of our city, such as the girls' schools, each of the adults' workplaces, our house, Nanny's house etc. and you can set it up so that you get an alert when someone reaches one of them. So, for example, you can set the app to sound an 'alert' when your DS arrives safely and it will display a message saying "Bob has arrived at Happy Street School" - you can set another for when someone leaves one of your marked places too, so you'll have a rough idea of when to expect him home, which is handy for those occasions when they get distracted and end up chatting/kicking a ball about in the playground for 15 minutes after the end of the day bell has rung!
It can be a bit intrusive if adults are using it though (for the adults, not the children!) in that occasionally my DM will say to me something along the lines of "What were you doing at Sainsbury's at 7:15 this morning?" without me having mentioned it to her! If you experience this, my advice would be to switch off your own location (you will still be able to view everyone else, it doesn't affect the app in any other way) and to explain that you've found it was really running your battery down! It does affect your battery usage (albeit not massively), particularly when someone else is using it to track you, as each time they 'refresh' it their end, your phone has to reconnect and refresh itself, just something to bear in mind.
Just make sure that your DS has enough data on his plan to keep the app running - I've found that on the rare occasion one of my DDs has run out of data and therefore the app hasn't been able to run, I've been quite anxious - more so than I would have been had we never used the app in the first place - just because I've become so used to being able to see exactly where they are at any given moment.
Anyway, it sounds as if you have prepared your DS well - it's always going to be a little nerve rattling the first few times you let them do something new, but I think you seem to have judged his age and the difficulty of the walk properly. Fingers crossed it all goes smoothly in the morning and you are able to relax, he (and you!) will get the hang of it in no time!