It’s not the fact he had a cold which would bother me. I agree, fresh air is very important, and yes, there’s a great deal of advantage to being outside generally. It’s the fact he’s only 13 months old. How safe is the garden? Was he being watched? Too much flesh exposed is bound to increase his vulnerability.
In our garden for example, we have foxes, mosquitoes. It’s big, so many places to hide, and on a hill. A nightmare in many ways.
I suppose by the time you get to 13 months, you’ve got a pretty good idea of how your children are with their children, and maybe shouldn’t have been a surprise. I would never normally advocate getting involved, but in this case, I’d want to double check.
I can’t imagine any of my children doing this to be honest.