Cześć, Yoddleman. Are you sure he waas given dexamethasone and not prednisone? Dex does not come in 20mg tablets, unless somehow it's different there in Poland. 80mg of pred is not a dangerous amount, and I have seen 80mg/day given to accomplish deliberate immune suppression. However, those types of steroids need to be tapered downward, weaned off so to speak, in order to avoid an immediate bad reaction. But that didn't happen, even though your grandpa accidentally had a very fast taper.
You asked about late symptoms. Immune suppression from 80mg/day can lead to infections. But when he was on only 20mg/day? Immune suppression is said to occur when *greater* than 20mg. So that's borderline, but you can be wary of any rise in temperature, etc. Sometimes, infections can be hidden.
1mg of dex is equivalent to around 7mg pred.
Hello~I would keep watching him for any adverse reactions, but I doubt if there will be any, but I am not a doctor. For peace of mind, I would also seek a second opinion.