My stepfather is leaving my property (Land in Michigan) in his will. Is this the same as a biological child? Is it necessary for him to adopt me?

The simplest and easiest and lowest tax way to leave you his real estate, is to grant a deed to himself and you jointly with right of suvivorship. I highly recommend having a lawyer draft the deed for you. If the land is valuable and, especially if he has other children, it would also make sense to have him do will disposing of all of his property so there are no questions when he is no longer with us. There is no need to adopt. Just remember, estates can bring out the best and worst in people; having everything in place is always best.