One imagines that Plod may have some thoughts on that.
I don't think they'd be pleased, but having mulled it over i'm not sure theres much they could do.
If he'd bolted it to the bike, then the bike would become a motor vehicle. Naughty.
However, worn on the back its a means of propulsion, but it isn't a vehicle in its own right, or part of a vehicle. It falls own the cracks between laws, and I cn't think of any applicable laws.
Grasping at straws they might try the catch-all old 'public nuisance' law (never been a copper in scotland, so I don't know if they have that law) but thats desperation measures and I doubt a charge would be forthcoming.
Of course, if he took off then it would become the Civil Aviation Authority's problem.