Everyone always says when you cheat, you cheat yourself:
so here, if he did not cheat it is a shame for him, and if he did then he has stolen from himself and those close to him any true enjoyment of a Boston finish, and every race or running club event he ever attends will know of him or he will have to wonder who knows and just isn't saying anything, he just potentially traded away a lifetime of enjoyment for himself
Once everyone has finished throwing accusations and insults at him in the pillory of the internet, maybe we can all remember that the real thing isn't that we can catch all cheats or prevent them, but we can do the right thing ourselves and pat people on the back who deserve it more often than we kick people who are down even if they deserve it.
That said, he does not seem like one of these people and no one is coming forward for him to say so, but most of us know people who if they told me they ran a marathon and they did not show in the photos I would believe them, might find it hard but I would. Those people are rare though and cheats are common. Pillory away but do not let this guy steal your humanity.