I would build a bug hotel. Collect as much wood, brick stones , Pots, straw, paper, and plastic together and give him a free hand. There are lots of ideas on the Children’s Garden’ wedbsite. Also any thing that will hold shallow water. We had 3 separate areas in the garden and it was amazing how much wild life collected around them. If you can get an old camera for him, then photographing the bugs, birds, flowers and veggies is a great hobby. He can create either a digital or paper album which will make a lovely keepsake for his future.
Favourite seeds for my children were Nigella and Honesty. The Nigella seeds heads, they called Cinderella coaches and the honesty, Fairy Purses.