Your child should be seen by a Doctor. A child (or anyone for that matter) can develop allergies at any time. Symptoms may or may not be connected. Does your child have a fever, runny or stuffed up nose? Keep in mind, we are now entering Ragweed season and this could also be a concern if you are near those forest preserves. Corn and other crops are also maturing in the surrounding fields.
Have you given your child Benadryl and have the hives responded to the medication? He should be taking something and you should see some kind of positive response to the medication. Hives can just be a precursor to a more serious allergic reaction, such as anaphylaxic so he should be treated.
If you are trying to narrow down the culprit and you think it could be the fabric softener, I would definitely re-wash any clothes & bedding that he could come in contact with.
Also, is he eating more fruit? berries, watermelon, etc.. ? Upon first ingesting these, he may have been fine, but he could have developed an allergic reaction so, please, when you do bring your child to the Doctor, possibly try to think of all he could have eaten over the last few days (newly introduced as well as old favorites) just to see if there is any pattern.
If you can't get your child to the Doctor, at least, head over to your trusted Pharmacist who can look at your child and give you their professional and free opinion. My Pharmacist has been my lifesaver, at times. It's good to get to know them.