Hi Ian
Welcome to MDs. I've been in a similar position when my kids wanted to give it a go for the first time. I'm sorry but I don't know your area so can't suggest locations but this is what I did.
What I did was ask at the local tackle shop(s) for pointers towards an easy location with plenty of small fish. What you need to do is get plenty of bites when you go with your son for the first few times and retain his interest.
I was lucky enough to find a couple which fitted the bill. One of them incidentally has a touring caravan park alongside it, which has a small playground. That worked really well, when they got a bit bored they'd go off and have a play and then come back and have another go.
Another tip, try to get a short 3/4m whip rather than a rod & reel for your lad to start with, much less hassle with tangles etc.
Hope you find somewhere and both enjoy yourselves.
AlanP