There are plenty of places with good footing, where a 10 year old would be perfectly safe. He'll probably climb down the rocks at 10 easier than I can at 60.
Or go there at high tide and have him fish from the bank.
If you want to fish from the pier, go there at a west tide, where he can walk the fish back along the side of the pier to the rocks, where you can go down and reach it for him. They are rebuilding the east side now, and you can't walk down that side of it.
You can also fish from the bulkheads at Sandwich. There's a cable railing there now that wasn't there when I was 10.