I've never seen fur on a buck's testicles, so he is probably okay, though he may sterile from the heat. Do they dig burrows or do you use above ground shelter for them?
If they have an abundance of weeds and grasses in their pasture, they are likely getting enough Vitamin A... Though dandelion greens are good for them anyway. I'd add some BOSS (black oil sunflower seeds, the kind you feed to the birds), starting with a teaspoon per rabbit and working it up to a tablespoon. You could also add ACV (apple cider vinegar) to their water at the rate of 1-2 tablespoons per gallon.
One unusual idea that many people have been having success with is to give them a crock of Celestial Seasonings Raspberry Zinger tea. We don't know why it seems to help, but many people swear by it.
None of these measures are going to help if the buck is sterile from the heat. Only time and cooler temperatures can fix that.