hi i have 3 dd's aged 4,3 and 14 weeks.
my girls love helping me in the garden, although it is harder with them helping to be honest.
like you i end up wih dirt all over the lawn, but the girls love learning how to take care of the plants. This year i dug them their own veg plot they grew carrots beetroot, tomatoes and strawberries. next year we are going to plant peas aswell.
I just think how much the girls are learning from being out with me and i think its worth the mess.
have you thought of giving your ds his own plot, to tend.
btw, looking after their veg plot keeps the girls away from my borders which are stocked with allsorts from pinks, lillies, lavender (which they love to smell), dahlia's etc...
another idea is to plant plants that will interest your ds, such as the lavender he can smell or ones that attract butterflies.
My girls have got so interested in gardening that they have handpicked plants to go in the garden and in the house, and they remain their plants. not mine. hth.