@Ironclad No, I can believe it. He just focuses on balancing his career while raising his son as a single dad. I have a 2 year old and a 1 year old. Even with my wife and I equally sharing parenting responsibilities, there's not much time for anything else. Imagine raising 1 kid by yourself, while doing all you can to succeed at work to provide for your child. When are you finding time to go out on dates with women? When your kid is 5 years old and asleep at home? When they're 10? Also, you're going to be telling those women you have a kid at home, how many women are willing to care for a young child that's not theirs?
What if the kid remembers their mom? Is there going to be resentment? (overwhelmingly yes, it's rarer that there isn't). Most mixed families (children from different parents) start when the kids are already grown a bit, cause it's easier for both parties to deal with an older child than a younger one.
Finally, sudden death is easy to accept explain too. I had a friend who died two years ago from stomach cancer, at the age of 30. He was already terminal when they discovered it. Another was diagnosed with throat cancer at 28. I find out the other day that there's diseases that can create aneurysms in children. Or it came about as an accident; car crash, building site accident, loose railing, robbery gone bad.
The details of the death isn't important to the plot, if it were, the author would've expanded on it. The most important details are; dad remarried young mother with two younger sisters, and died shortly after, cause the story is going to be about the MC's stepmother and stepaunt's/sisters and his interactions with them.