>His estimates of other people tasks are just ridiculously short while for his own (which rarely happens) there is a huge amount of padding ( a good example is for some basic queries about 1 week lol).
>We have tried several tactics to change him but none of them worked.

If I encounter such types I switch into very old mechanisms:
Have one person estimate all the tasks, but let all others pick their tasks knowing the estimates before he gets his pick. That way he will do his level best to get correct estimates, as the job where the greatest discrepance *not* in his favour is likely to be left for him. Won't work in areas with special knowledge, but variations on that theme can be brought into play.

regards

thomas