Iron Man and Howard Stark would likely start with a bit of shock. Howard might be amazed at the technology Tony has created, and Tony would be in awe of seeing his father alive (if it's that kind of scenario). They'd probably exchange some witty remarks, with Howard being a bit more old - fashioned in his speech. Then they'd start discussing technology, with Howard offering some of his classic insights and Tony showing off his modern advancements.
There's no official indication in the main Marvel canon that Howard Stark reads Iron Man fanfiction. Howard was a brilliant inventor and businessman, mainly focused on his scientific and entrepreneurial pursuits. Fanfiction is a modern and often fan - created concept that might not have been part of his world view. However, in some alternate universes or fan - speculations, it could be possible, but still not a common or established idea.
Well, Howard Stark might have a range of reactions. He could be in awe of the advancements, or he might feel a bit left behind. Maybe he'd also have some fatherly pride and a desire to have been more involved.
He might be confused at first. Howard Stark was from a different era and fanfiction was not a thing during his time. But if he understood the concept, he might be flattered that his son's story had inspired so much creativity among the fans. He could also be critical as he was a perfectionist, and might not like some of the liberties taken in the fan - created stories.
One possible plot could be that Iron Man, Tony Stark, travels back in time and meets his father Howard Stark. He could try to prevent some of the events that led to Howard's death and in the process, learn more about his father's early inventions and ideas. Maybe they collaborate on a new piece of technology that changes the future of the Stark family.
Maybe it's a tale where Howard Stark is somehow able to observe Iron Man's adventures from a hidden vantage point, and his reactions shape the story. Or it could be a story set in a parallel universe where Howard Stark has a front-row seat to all of Iron Man's heroic deeds.
There's also 'Howard's Shadow'. This fanfic focuses on Tony's struggle to live up to his father's reputation. It has a lot of flashbacks to Howard Stark's days, which gives great context to the Iron Man story. It's really interesting to see how Tony navigates the challenges while being in his father's shadow.
In many fanfictions, Howard Stark's early tech designs are a major influence. For example, Tony might find an old blueprint of Howard's that gives him an idea for a new Iron Man upgrade.