Technically, the survival of the father in the prequel is not guaranteed by his presence (or apparent presence) in the first story. The most obvious way of accomplishing this is to have the first story be "it was all a (pleasant) dream".
This could be presented in the title of the first story (e.g., An Heir's Choice: The Dream of Argoth) in which story "dream" could refer to the father's prophetic dream which started the quest and to the father's aspiration for his son (e.g., that he would be a hero like he was) but also to the story itself in the context of the prequel.
For example, the father's quest could be to destroy his ancestor's crown to end the threat of a great evil that entered the world by that ancestor's choice. Near the end of the story, the father could be spending the required three nights in the Chamber of Dreams before entering the King's Vault that holds the crown. Perhaps halfway through the story, the father discovered that ending the evil would require his death, and the first night's dream confirmed this; putting on the crown and taking responsibility for the ancestor's choice ends the evil and his life. The second and third nights' dreams could be presented in the text as a summary of the first story, where destroying the crown by casting it to the ground, rejecting the benefit from the ancestor's choice, banishes the evil for 343 years. The prequel could end with something like:
At last he held the crown, the weight of centuries of evil felt strangely light in his hands. He made his choice.
With such an ambiguous ending, the first story could be actual history (as revealed by the dream) or a future that might have been. (Incidentally, a third novel (e.g., An Heir's Choice: The Hard Path) could present the alternative future in which the evil was ended rather than banished.)
Even with the title hint, it might be difficult to write such stories. However, unlike the "it was all a (bad) dream" cliche, the detail and attractiveness of the story would make it a very powerful, real temptation for the father and less of a cheat to the reader.
Another possibility would be that his father is not his father but rather, e.g., his uncle. In the prequel the father's brother could witness the father's death and swap identities ("my brother gave his life for me and for all the Land of Crestern"). The first story might present hints that his father is not his biological father, but this fact need never be revealed in the first story. (A third story might reveal this fact.) A heavy emphasis on paternal and fraternal love in the first story might strengthen the end of the prequel.
Resurrection is a third possibility. The father might die with the full assumption that he is leaving his son an orphan, but he might be brought back at great cost (which might also be why this fact is never mentioned in the first story but it might be hinted by the father saying things like "I owe her my life and more", "life is not the highest good", "one cannot always see the beginning or the end from the middle").
Retcons are difficult to write well. Even with hints and ambiguities in the first story, the reveal in the prequel can easily seem like a cheat or just an unnecessary confusion to some readers. Making the first story clearly matter (using the attractiveness and reality of the dream to emphasize the temptation, using the brother/uncle's love and dedication as an adoptive father in the first story to emphasize the fraternal love in the prequel, or using real death (with fear, pain, and grief) and the price others were willing to pay to bring him back to avoid a perception of insignificant death) is one way to reduce audience disappointment.
Of course, none of these actually solve the problem of perceived risk since the readers of the first story will assume that the father survived. As the other answers state, the general outcome of success and survival need not be the primarily source of dramatic tension (happy endings are commonly expected). Death and outward failure are not the worst fates possible. In high fantasy, personal honor is likely to be highly valued ("Gawain and the Green Knight" has Gawain survive but at the cost of dishonor even if he was judged more honorable than other knights; or quoting Aral Vorkosigan's statement in A Civil Campaign (Chapter 15), "There is no more hollow feeling than to stand with your honor shattered at your feet while soaring public reputation wraps you in rewards. That's soul-destroying. The other way around is merely very, very irritating.").
How the character survives and succeeds (what cost, what heroic choice or determination, what clever device, what unexpected intervention, what flaw in the villain, etc.) can easily be more significant than the outcome.
In addition, when one is drawn into a story, facts and assumptions can be forgotten in the immediacy of the action as one's perception is guided more by feeling than by rational consideration.
Finally, dramatic tension is not the only reason people continue reading a story. For example, satisfaction in the felt truth of a story need not depend on the unexpected (people do re-read stories).