what speed does he move?
how heavy are the carts?
what is the resistance of each of the wheels?
what kind of surface is he on?
what kind of shoes does he wear?
is is level ground or uphill?
how many steps does he take?
whats his average heart rate during on trip?
how long does each trip take?
Sorry, yes we can work it out. But its very complicated and not at all usefull. Lets just say that it doesnt count as exercise. If your heart rate is not high, then your not doing anything.