Maybe a set of a hand fork and a trowel would be a good midway point - small enough to handle but actually be able for some work? As in an adult fork and trowel, but they are still fairly small. With something like a cheap spade or fork from Lidl or similar for if he wants to tackle some "proper" digging. I know that we got the toddler (she was 2 at Christmas) a set of hand tools in her stocking- from a GC, and kiddie sized - and I got a Lidl spade, brush and rake last summer when I saw them (she loves the brush but the other 2 are beyond her yet). But the hand tools are already in use in the garden and she was insistent that we bring them when we went to the plot so far (but she isn't interested in using them there yet - maybe later in the summer or next year :O).

In the meantime - plenty of large seeds to help you sow and fast growing things to eat - definitely grow at least one cherry tomato plant and some peas to eat straight from the pod!