Whoever Billie meant that "now suck it" for, it's cute if he wants other people to suck the lollipop in his place.... (I say in his place since he should supposedly do it, given Adie has posted another one). It could also be a way of ask us to encourage him / taking us as witnesses he will do it (so he says it for himself), or do it with him on our side. Or yeah it could be meant for Kevin after that "you know something about sucking?" I can't find the video for.
After analyzing Billie's songs, trying to analyze a 3-word phrase of his on Instagram, and it ain't easier