I was up there a week ago - in the mist.
We went up the back track and walked from Gowrie Park township.
The track is not a gravel road anymore and in places very beautiful with sassafras and gums, manferns, musk and other minor species. (I posted some pics
http://bushwalk.com/forum/viewtopic.php?f=3&t=5909&start=30 and previous page.
The day I went up we met some others and I would say the ages ranged from 52 - 75, so none were chickens and all of us looked fit and able.
This should be an easy paced good walk for your dad - you say he is 'extremely fit'.
On the other hand the '3 Peaks' are a lot more arduous IMO. The climb down from Claude and then back up Vandyke is quite steep. Its a beaut trip though. The track probaby mostly would be a bog judging on the amount of rain we are having and more to come.
The front track is one long steep slog - I never enjoyed it, especially coming down. But it is a scenic one.
As to the weather in May.
Its a time when we get anything but is probably more settled in as much you are less likley to start in sunshine and end in rain or snow, or any combination of. Check the forecast and work with that.
I'd say go for it and enjoy.