Having read this a few times, I don't think he's a sexual predator.
I do think he probably has an alcohol problem, and that's a big issue in itself.
He's been convicted of drink/driving - never a good look on someone who works with vulnerable people.
He supervised a child trying alcohol and later that evening, whilst that child remained under his supervision, he thought it was appropriate to get bladdered himself, rather than remaining sober.
Reading between the lines, the girl was his (then?) partner's child, so he was in a sort of stepparent role. Parents do drink when their kids are home, but I think very few choose to drink when their children are ill or incapacitated. If he was supervising the girl's first taste of alcohol, he didn't know how she would react, and he should have stayed sober in case she felt ill/funny afterwards. Parents who drink at home tend to choose their moments carefully.
A normal teenage girl walking in on a trusted adult man having a wank does not react to that by initiating oral sex. She probably screams or shouts and runs out of the room, highly embarrassed.
A teenage girl who reacts by initiating oral sex is deeply troubled indeed, and if she is effectively living with a social worker stepparent, I would expect him to have identified those troubling behaviours beforehand and to have raised all kinds of safeguarding alarms. It doesn't sound like she was on anyone's radar until she reported this incident to the police. It sounds like she should been receiving support, and given his pseudo-parental relationship with her, he should have picked up on that.
I accept that he may have been shocked to have been caught masturbating and it might have taken him a few seconds to process what was happening and to push her off, but I question the judgement that even led him to be in that position.
The drinking at home that evening was a bad call. Wanking in a communal area of a house with anyone else in it apart from a partner was a bad call. I mean, even if he had been living with adult housemates, wanking in the living room would have been a bad call. If it's not your private home, you watch your porn in your bedroom only.
On the basis of the information provided, I don't think he had a thing for young girls, but I do think he makes extremely poor decisions, and that's not a quality you want in a social worker.
I really hope this girl got the support she needed. I do worry that she had problems before this incident, and it only exacerbated those problems.