Could the father's name be Gulielmus?
In this context it should be
Gulielmi (genitive).
At a stretch someone might argue that it ends
_mj and means
_mi but in my opinion that is highly unlikely.
I can't remember ever seeing a
j standing for an
i at the end of a name.
I also don't feel that the beginning of the word looks like a
G, but that's only an opinion.
You might wish to search nearby pages for a similar looking name? Or, search for a William to see how it is written?