It depends on your location in the world what parasites and diseases are to be found so I will be mentioning the ones in my area of Canada as examples.
First things that come to mind when allowing a dog to eat from the ground outside is:
If your dog isn't regularly treated for worms and has some he will be re-contaminating himself by eating kibble near his feces or from other animals feces. If he eats any slugs/snails or feces from these invertebrates there is a possibility of catching fox lungworm.
Leptospirosis is obtained from drinking urine of infected animals (foxes, raccoons, squirrels) or contaminated water sources like puddles. This disease is zoonotic (transmittable to people) but preventable through vaccination of your pet.
Risks are low for him to catch something in your back yard unless there's high volume of critters and other pets running through, that being said I'm sure he'd love a foraging toy!
Pet stores have MANY cool foraging toys to slow down his eating and make him use his brain! Some examples:
These are some of the MANY products that will slow down his eating and make him work for his food in a cleaner environment.