While it's true that he's Salman Taseer's son, the fact also is there that Aatish, being at least nominally Sikh, is a kaafir. I find it sad that you'd believe the allegations of a kaafir against a Muslim.
As for the drinking.... well, the PPP is the party that cheered for its leader (Zulfikar) when he admitted in public that he drank. Based on that one can only assume that the PPP and its supporters are either drinkers or pro-drinking.
Did you wear Mullah Fazlurrehman's topi today?