We've had (slightly more graphic) discussions of this elsewhere, and a few points don't line up for us.
Alfie has, according to the papers, "not gone through puberty yet". Certainly his voice hasn't broken and he's absolutely tiny. So it raises a couple of questions - certainly he could have had sex with this... err... "girl". Most males meet the qualifying standards for this pretty much at birth. But whether he could have fathered a baby?
Not to get too graphic, but I seriously doubt that, at his size and developmental status, he could have produced "the required substances" in sufficient amount, containing a sufficient amount of healthy, mature sperm. I know that, in theory, it only takes one, but there's a very good reason we make 3 million at once... The only counter to this is that Alfie's father has produced 10 children - but this isn't really an indicator of anything aside from the average distance between the ankles of local women.
So we have quite a collection of really, really long odds - including the mother being at the exact right point on her cycle.
I don't think it's especially likely - though not improbable - that Alfie is the father. In fact I suspect he's a very convenient scapegoat to cover up some familial abuse. Instead of blithely accepting it, his family need to demand a paternity test STAT.