Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am very sorry to hear about poor Lexis's hives on his legs.
The most common cause of hives and welts is an allergic reaction.
This is usually related to an insect bite or sting (perhaps mosquitos or fire ants as he went through tall grass?). Dogs rarely have a reaction to stinging plants, like poison ivy, but there are some that they do react to such as Wandering Jew plants. Sometimes we never figure out the cause because the bite/sting is covered up by diffuse swelling.
Treatment is based upon stopping further allergic reaction with antihistamines like Benadryl and sometimes cortisone if the reaction progresses.
Is his breathing and activity level normal?
Any vomiting or diarrhea?
You can give him Benadryl (diphenhydramine only don't use the combination products with decongestants or acetaminophen as they can be toxic for dogs) at 1mg to 2mg per pound or one 25mg capsule per 15 to 25 pounds of body weight orally every 8 hours. 2-3 of the 25mg tablet size of Benadryl every 8 hours for a 65 pound dog would be appropriate. You'll need to give it for at least 72 hours (3 days) to allow the allergen time to clear. If you stop too soon the reaction will recur.
Dogs take a really high dose of antihistamines compared to people, so don't let the amount worry you. They require more than we do. This may make her a little sleepy.
I would give him a cool water bath with an oatmeal based shampoo to soothe his skin and remove any trace of the allergen in case this is plant based. The cool water will close skin capillaries and reduce inflammation and oatmeal reduces inflammation. Lather gently, let it sit for several minutes and then rinse with cool water.
And then you'll need to watch him closely for any progression of symptoms. That includes difficulty breathing, vomiting or diarrhea.
The swelling/redness/bumps should resolve slowly over the next 12 to 24 hours.
In extreme allergic reactions they can get shocky and collapse but you should see difficulty breathing and vomiting/diarrhea first. Those types of severe reactions are very rare. Most reactions are managed just fine with antihistamines alone.
Because you know now that he has these types of reactions you should keep Benadryl on hand at all times.
If you notice progression of his symptoms, especially difficulty breathing, vomiting or diarrhea, don't wait you should have him checked on an emergency basis.
If you'd like you can have a veterinarian check him now. They can give a cortisone injection which will lessen symptoms faster. But if he seems to be doing well otherwise then I would just call your regular veterinarian tomorrow and let them know so it is on his record for future reference.
Please let me know if you have any further questions.