Question

Unable to add ggf because I don't know DOB.

Answer 1

In general you can use a rough year estimate, and then mark the date as uncertain. Sometimes that is all we have. Use only the year in the field for birthdate. Please include an estimate for the location (eg country) as well. That really helps others looking at the same surname.

But make sure you have at least one real source showing the link others is real (otherwise the date is uncertain and the profile unsourced).

Answer 2

Estimate his date of birth using information of other family members.

If parent and child birth dates are available then estimate his as halfway between them.

If only parent or child is available estimate his as about 20 years after or before.

Answer 3

Do you have his date of marriage?  Then go back 21 years and mark the birthdate as uncertain/estimated.

Answer 4

Do you have his date of death and an age at death?

Year of death minus age at death will give you a rough YOB.