I am adopted and have info on my maternal side, but not paternal.

I recently got a match through AncestryDNA with someone saying we were close relatives. This man and I share 2086 centiMorgans in our autosomal and 20.6 centiMorgans in our X chromosome.

We uploaded our results to Gedmatch and it just confirmed that information.

I uploaded my daughter's results to compare to him and she shares 1100 centiMorgans with him (but no cm on the X). We are trying to figure out with certainty if we actually are 1/2 siblings.

Is his father my biological father? And if so, why do we also share a small amount in our X chromosome, since we definitely have different mothers?

Everyone we have asked seems to confirm that we have to be 1/2 siblings -- is there any other way we can be sure?????