I'm also unsure. It doesn't look good but we don't have all the facts yet. There is obviously a lot of political pressure to investigate but that doesn't necessarily mean that any rules have been broken.
If he is fined, then I think he will absolutely have to resign, no question. It would be incredibly hypocritical to stay in post, and if he doesn't go of his own accord, then the Labour Party needs to sack him. There is no room for any flexibility on this.
If the police conclude their investigation and confirm that no rules were broken, then obviously, he can continue. I don't know enough of the detail to know whether or not this is likely to be the case.
Personally, I would be quite surprised if he was found to have broken the rules given what he said about Boris. If there was ever any shadow of a doubt that he might get caught out himself, you'd think he might have been a bit more circumspect in his response. He doesn't strike me as a stupid man, or as a massive risk taker, but who knows?
If he is fined and he does step down, I guess that will make Boris's position rather difficult, especially if there are more fines for number 10. Labour could at least say that Starmer had taken responsibility and fallen on his sword. If Starmer is fined and stays in post, though, the Labour Party clearly won't have a leg to stand on with regard to any further comments on partygate. And the electorate will understandably conclude that they're all as bad as each other.