You can't go wrong with the 85mm 1.2 or the 700-200 2.8L II.
The 85mm is the de-facto portrait lens and at 1.2 in combination with your mk iii you could almost shoot in the dark
. At 85mm it can also be a decent walk-around lens on a FF a bit too long on a crop sensor. Also has one of the best bokeh of any lens.
The 70-200 is no slouch as a portrait lens also and is tact sharp even when wide open. It of course is a much more versatile lens, it can be used for walk-around/portraits, indoor/outdoor sports, parties, landscape and even as a faux macro lens.
Both lenses makeup 2/3 of what they call the holy trinity of wedding photog glass
. I have had my 70-200 2.8L ii now for 3 months and it has pretty much stayed on my camera the entire time. I've used it for everything stated above except for Macro, since I already have a dedicated lens for that.
Congrats on your new camera, I think it's cool that you and your dad share this hobby. I would be stoked if my son keeps up with photography when he gets to your age. He is already shooting at 5, so we will see