Does examining the alleged son, whose paternity the father doubts, a year and a half after his birth, constitute a disclosure of God’s concealment, and is this act forbidden?

If the doubts are mere suspicions, it is not permissible to disown the child or to perform a paternity test. However, if there is certainty or strong presumption that the child is not yours, then you must disown him through li'an, and your concern for concealment should not prevent you from performing the test, because disowning the child in these cases is more obligatory than concealment.