@Anderton
You do not have to keep his secret for him. It is totally wrong for a child to feel they have to keep their existence quiet in order to preserve the feelings of the family that their parent now has - you are also part of that family.
I would make contact gently and see if anyone would be willing to do an Ancestry test to confirm the relationship.
In my similar situation I made contact with two half siblings. I felt like I was viewed with suspicion and chose not to pursue a relationship. My existence was acknowledged though and I believe every child has a right to that.
I was in a similar situation, too.
In hindsight, I would have been better to have involved a social worker to mediate rather than do it myself, as then a lot of issues could have been addressed beforehand, about feelings, expectations, the past, the future, etc.
I was adopted for reasons. The family dynamics were difficult and there was a great deal of narcissism and drama, which I unwittingly got drawn into after I made contact. It has been a difficult journey, and I had to distance myself somewhat to maintain my own peace.
Overall, I am glad that I know my birth family, especially my siblings, and have got to know family histories, etc, which sort of helps place me in the world.
I found that my siblings knew all about me, which was a surprise, but that in the extended family, me and my other adopted sibling were dirty secrets, and I was never allowed to know my paternal grandparents, who were from the “respectable” end of the family.
I would have liked to have known them, and also to have my existence acknowledged in the wider family. MY wider family. which I had a right to know about, and maybe to know, and possibly even to join. (Ah, but there's the romantic ideal again)
I would advise adjusting expectations, if you do make contact with wider family members, and work on coming to terms with your father's refusal to know you first. The suggestion upthread of involving a third party is, I think, the way to go.
Good luck. 