Add him as a possible or probable father – as many as you need while the family works it out. A "maybe" or two? Very normal! Under their Parents, press What are you trying to fix? and choose the option saying you're not sure who one of their birth parents is. For example, if family stories say Grandad's dad was either Jack or Ron, add both as possible fathers. When you find out, make the right one certain.
Where to find it
Person page (Safe edit on) → under Parents, Partners or Children → What are you trying to fix?
Only people with the "Add people" and "Remove" ticks, with Safe edit on.
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