-- Last Updated: Mar-21-13 2:34 PM EST --
...either your size or your father's dimensions. But that's not such a big deal with the kayaks you are mentioning.
I noticed that both the Dagger and the Perception are even shorter than your Aruba while being about as wide. Based on that data, I'm thinking that they would be slow and track rather poorly.
Also it does not appear that either has built in floatation fore or aft. Even in situations of casual usage, you really do need floatation in both ends of your boat as well as perimeter lines.
The Dagger Axis looks like a big step up but appears to only have floatation in the rear, which is still a bunch better than none.
Hopefully someone with more experience with these boats can provide you with better information.
Oh, and BTW, I'm sure that you and dad are using properly fitting PFD's? :)
And a PS: It's important that your dad gets used to capsizing and knows how to get back in his boat. This is a water sport and everyone is going to flip sooner or later. The first time I capsized was on flat water when I was just messing around. Now I think I'm doing wrong if I don't capsize:)