There are very few fathers present in About A Boy, which is almost entirely peopled by single mothers... and Will, of course. The exception to this is Will's best (only) friend Christine- a former girlfriend, naturally- who is now happily married with two children. Will is bemused and faintly horrified that stylish Christine can be content with her messy kids, toy and laundry-strewn house, and rumpled dad-bod husband, but they are the most grounded and happy people in his world.
Marcus' home life is abysmal: his mother is a hippie/activist type who is prone to bouts of depression. Marcus frequently comes home from school- where he's bullied unmercifully- to her sobbing on the couch, and once to find that she's overdosed on pills. His father is not in the picture... he shows up once during the film, at Christmas; he gives Marcus a package of sports socks as a gift. It's not evident that Marcus' father and mother were ever married, or even a serious couple; his dad is another aging hippie/free spirit, which can be taken to mean that he's irresponsible and useless.
All in all, this movie presents a pretty dark look at the results of broken families and fatherless homes. The single mothers are tired, stressed, unfulfilled and unhappy. Marcus' mother Fiona is a self-pitying mess, completely overwhelmed by life. The single mother that Will ends up dating has a son with anger issues who reacts violently whenever his mother brings a man home, obviously feeling threatened and insecure. Marcus palpably hungers for a father figure, to the point where he inserts himself into Will's life despite Will's obvious lack of enthusiasm. And, despite Will's equally obvious deficiencies, Marcus asks him to date his mum because he longs for a family which isn't so broken and disfunctional.
Will himself is the product of bad parenting. From something he says, we're given to understand that his father was an alcoholic; he obviously provided for his son materially but was otherwise neglectful and remote. Being given wealth and indulgence but no fatherly affection or guidance has left Will selfish and unmotivated, dodging all forms of maturity or responsibility. This, I suspect, is why he engages in so many short term relationships: anything else would require time, effort, and emotional involvement.
So, against all odds, About A Boy has a rather conservative message: in tact families are important, and children are very often collateral damage when their parents can't- or won't- make it work. Also, children need male guidance (fathers) in their lives as well as female, and familial responsibilities are good for the character of the man as well as the child.
A clip from the film (language warning):