This is because the only time I do see him acting like a parent is when he acts like a dimwitted homophobic arsebucket father who resents someone else's son for turning out gay and implying nothing at all about him. I also remember that the situation was resolved by the fact that instead of the bog-standard admission that he was out of line to rant about Lawrence's "kind", he made a sheepish admission that he had no reason to do anything because he was not actually a part of the boy's life. It's at this point that we should remember that he's also not part of Molly or Gayle's lives either. I can only remember him talking to Molly once and that's just to back up Connie.
The reason for this is that just as Molly and Gayle are Lynn's first attempts at ridiculing teenagers for not especially caring about the crap that got her hepped up back in the Pleistocene, Greg's purpose is to provide Connie with money, status and a house next door to the Pattersons. He has no other function because otherwise, people might get the idea that people who aren't the Sainted Pattersons might have an idea of what they're doing being parents. If they start thinking that, they might actually follow Molly into her darkness and ask themselves "What the Hell is so God-damned great about that idiot Elly and her irritating family in the first place? Why are these dirt-ordinary people treated like they're royalty?" and be dark people embracing darkness that is dark.