He was being sought for extradition to Sweden, and was seeking political asylum in Ecuador, I assume due to lack of extradition treaties with Sweden. I believe Ecuador wanted to give him asylum, but the UK decided to hand him back to Sweden. So if he leaves the embassy, the UK will arrest him.
He’s pretty much stuck there until either Sweden or the UK decides they don’t want him arrested, or until he decides getting arrested is preferable to living in the embassy. There isn’t much chance of him being smuggled out.
I just read it was for four sexual assaults that he was wanted.
I have no idea if it’s true or not.