What's more, management recently gave him a grilling for his lack of progress on said giga task. This dev is competent, willing to learn but completely demoralised by the situation and worries about being fired... I've politely communicated publicly to management that their expectations are wildly unreasonable and praising the progress anonDev has made.
What more can I do to stand up for this individual so that this doesn't happen next sprint?
With a week of progress logged it might be easier to steer the conversation with management towards the preparation of a realistic estimate for the remaining work.
If this is not something management appreciate or are interested in discussing it is likely the dev in question could find more pleasant employment elsewhere.