Since I am constantly in touch with teenagers, I can tell you that they sure have a lot of confidence. Sam's confidence was primarily because of a successful performance infront of an audience like the bikers.
Again, I understand how confident (& overconfident) teenagers can be & that the concert was the reason for Sam being "full of himself," & it's understandable. I still doubt that this would sufficiently empower him to follow the course he did. This is where, for me, the necessary suspension of disbelief became overused.
I don't think Sam's breast handling skills or sucking skills were supremely praised at all in the scene. Also I think that only happened when Liam was talking with Sophia and so Sophia couldn't immediately stop him. The immorality of the situation could be a reason for the turn on and not the very action by Sam.
He took to his task like a duck to a hot tub & Sophia's reactions were enough to convince the audience that he was no fumbling boy but rather someone who seemed to know exactly what he was doing.
Most important is that Sophia has constantly been teased and edged and then left just like that. Her body will feel good with touches like these and her longing because of being denied starts Playing a huge factor. Add to that the wine which was also mentioned as a cause for what is happening.
This I can believe up to a point but it takes two to tango & the use of wine/alcohol is becoming a trademark for an adult who seems to have near-zero tolerance for the stuff. She knows what it does but carries on anyway.