Mmm, you are being unreasonable, so is he, also fairly ignorant which is worrying.
Dog poo does not make anyone blind. Poo does not do that.
Worms that may be present in canid poo and also feline poo, CAN cause blindness.
Poo containing worm eggs needs to sit for between 10 and 21 days, at a constant temp and moisture level (ie not freeze, bake or dry out) for those eggs to hatch and pose an infection risk for someone who consumes the poo. (Yes, you need to eat it).
Worm your dog. Then the risk from your dog is gone.
Pick up poo straight away, this also removes the risk immediately.
Foxes WILL have worms, you can't worm foxes. Therefore, if foxes poo in your garden, and your child comes into contact with it, thats the risk of blindness from toxocariasis (ditto cat poo from unwormed cats).
Keep the grass short. Pick up poo.
Chuck hot water down, no bleach (it breaks down into ammonia and will attract animals back to mark there, foxes mark with shit!)
If you want to, fence off a bit for the dog to poo in and he doesn't go in the child play bit unless supervised.
You ABSOLUTELY can train a dog to poo in a specific place, it does take work, thought, supervision and effort though, from everyone who deals with the dog.
If you try to teach a dog where NOT to poo by taking them to it, pointing at it and saying no and frightening your dog, you just teach him that you + turd = scary, and you may teach him to avoid shitting near you, avoid you finding shit (they tend to eat it to do this, or hide to shit).