Your information's a bit scanty.
Do you mean that his wife's maiden surname was Steven?
There is a death of a Margaret McDowall, other surname Steven, aged 80, mother's maiden surname Beggs, in Glasgow Milton in 167. You could look at that death certificate and see what it tells you.
There are three recorded baptisms of Margaret M*cdow*l between 1791 and 1801. See screenshot.
I looked to see if the one baptised in Port Glasgow had a brother John. See second screenshot. You could to to
www.scotlandspeople.gov. and try a similar search using the names of the other couples who had Margarets baptised then.
Bear in mind, however, that the absence of a surviving baptism record does not mean that there was no child, as many records from the late 18th century have not survived.