First, he may be allergic to the grass. Second, I know my guys do not like the grass (or anything else) when it's raining or just afterwards. It's kind of Ewe, it's wet, get it away from me.
You could see if it's grass in general or specifically your yard by taking him to grass somewhere else and see if he's scared. If it's just your yard he may be afraid of whatever caused the hives and not the grass. If that's the case you'll have to try to eliminate the cause and then retrain him to like the yard by going out with him, playing and of course TREATS!