If you have thoroughly searched Scotland's People and found nothing, the overwhelming likelihood is that the record of William's baptism, if it ever existed, has not survived.
If it's not on Scotland's People then, realistically, it is not going to be on FamilySearch (or on any of the commercial web sites) because they only have indexes to the originals, all of which are on Scotland's People.
You do, from time to time, find so-called 'records' on, for example, FamilySearch, which are just guesses based on a different record and various assumptions, for example they might have found a marriage, and they invent a birth 25 years earlier for the groom and 21 years earlier for the bride.
There is an outside chance that there might be a stray register that escaped in 1855 when all the Church of Scotland registers were collected by the Registrar General. Or that there might be a surviving register of a dissenting church that has not been digitised because permission to do has not not been forthcoming from the congregation that owns it. Or there might be a register of the Episcopal Church lurking somewhere in a kirk or in an archive somewhere. But it's very unlikely, and it is going to be extremely difficult to find and check even the very small number of such registers that do exist.
So I think that you may have to accept that William is one of the large numbers of people (some say as many as 30% around 1800) whose baptism is not there to be found.