It's over 3 miles away. We've done the route and some of it is on a fairly busy road (in rush hour) but there is a cycle path/shared pavement and he can cut a lot of it out by cutting through the park and along a back footpath.
I think he will be fine, he needs the exertion and it will be good for him. However a couple of friends have done that wincey/gritted teeth thing when I've mentioned it as if it's terribly irresponsible.