>We have a daily online "scrum" during which I repeatedly asked him how it's going, and if there were any blockers. He replied, "I'm working on it". I knew it was a significant task so I didn't expect it to be done quickly, but I did expect far more progress than what was delivered.

Perhaps what you need to do is simply expand the scope of your standup to include a 30-second description of what was done yesterday, beyond merely reporting blockers.

If that turns out to be "I was not able to work on this project yesterday" at least you have visibility into the situation.

If the summaries are a lot of excuses, at least the picture becomes clear.

If the person refuses to communicate, don't settle for that, push until you get a meaningful answer.

Hopefully the summaries will be more meaningful, ie "spent most of the day banging my head against the wall over issue X but finally solved it by applying Y so am now able to move on to step Z"

It's not worth arguing about if this does or does not fit some other organization's idea of what a standup is for; process needs to fit the needs of your project, and if it presently does not, then it needs to be modified until it does.