He might. The laws of descent and distribution do provide some circumstances where an adopted child can inherit from his natural parent. The code section is 755 ILCS 5/2-4(d). The language isn't all that easy to read, but here's a link to the page where it's located if you want to try for yourself. http://www.ilga.gov/legislation/ilcs/ilcs4.asp?DocName=075500050HArt%2E+II&ActID=2104&ChapterID=60&SeqStart=3700000&SeqEnd=5000000
You probably should just talk to an attorney in the county where the estate will be probated. That is most likely the place where his father lived before he passed away. Acting quickly would seem to be a prudent move too, particularly if the father had significant assets. My office is near Gurnee if you wish to call. My phone number is 847-623-5900 and I'll be in until around 2:00 pm today.
Please be sure to mark the best answer to your question. My answers are general and do not form an attorney-client relationship. I'm happy to talk to prospective clients in my areas of concentration and geographical location.
If your husband's father's parental rights were terminated in an adoption proceeding, then your husband is legally not an heir to his biological father's estate. He could, however, be a legatee if his bio-dad mentioned him in his will.
Children who were placed for adoption have the right to inherit from their adopted parents and their biological parents. If the child has not been disowned in the will of the biological parent then that child can challenge that will in probate court.
Have your husband contact an attorney in your area to discuss this in person.