Wow that is very scary, so glad you rushed him to the Vet right away!
I have been there 3 times with Peanut- He was stung the same way, by stepping on them in the grass. I started to noticed bees/ wasps were always diving in my grass or hovering on top- DH looked around for a pesticide to treat the grass. We found a pet friendly product at Home Depot. We have been using it since the last time he was stung, since we have been using it- never do I see bees/wasps in my grass, or even around my grass!! I would highly recommend this product if you see them in your grass!
EcoSmart 10 lb. Lawn Insect Killer Granules-33114 at The Home Depot