He should be fine. My two have been on there from about 3 on balance bikes and now at nearly 4 and 5 they’re pretty confident on their pedal bikes. As above, so long as he’s OKish with his brakes he’ll be fine. If he’s not he’ll soon learn to use them 🙂
Fireroads are a good option too, and the playground is good. This time of year is a good excuse for a post ride hot chocolate from the cafe too.
If it’s a 40 minute drive then I’m not sure I’d bother though, there must be plenty more local.
Oh, if you’re going to come over I’d try to pick a dry day – haven’t been on the green for a little while but in general over there when the trail wears you get some decent sized stones uncovered, could be a little slick for tiny tyres and unsure braking. You don’t want him coming off and deciding he doesn’t like it.