install a dash cam in both vans, with sound and maybe one that shows speed if possible.
you could speak to the bosses and ask them to tell him its a new rule that it must be on all the time.
you can buy them on ebay very cheap, i got one for about £16 - if your bosses wont pay for them, at least its only £30-odd out your pocket, but if it makes him start to take more care because he knows hes on camera, its a good deal.
i realise it may not stop him eating and using his phone, but if he thinks he can be heard he may do it...