Brilliant!

maybe look into the Korg MS-20 Re-issue?

Well, if the computer distracts him too much, I'd recommend hardware, as well. I personally find the MiniMoog the easiest to understand from a layout perspective.

I think hardware would be the ideal but for the sake of balancing the investment with the likely return I'm going to start him with WebModular and then see if he still feels analogue synthesis is what he wants to mess with.

After that we will consider some of the other options, so thanks again for all the suggestions!

I found some old intro to synthesis videos on youtube the other day (also found on KVR afterwards). These may be useful for your father to get the theory, although warning - there's a lot of jargon in there:

http://www.kvraudio.com/forum/viewtopic.php?t=338773

The best way of learning synthesis it's real time manipulation of the different parameters, so as others, I would recommend an Analog/VA hardware synth, specially one with no presets, or hidden menus and stuff.

Synths like Microkorg or Micron can be really difficult for people with little synthesis theory, so beware not to buy an overly complicated one.

I'm just 31, but I hate complicated stuff myself as I don't have time nor patience to learn complicated interfaces, and let me tell you I'm really happy with my Minibrute. I learnt synthesis on a Juno 106 (probably best synth ever for learning synthesis), but I wouldn't recommend this synth (I don't have it anymore), due to it's poor reliability, as most synths from yesteryear.

Why don't you take him to a Guitar Center (or whatever music store you have available) and play a minibrute for a couple of minutes? Maybe even a humble Monotribe could have your dad making weird noises and trippy sequences for months as a lot of people I know!

And don't forget to check old TweakHeadz (RIP ) tutorials about synthesis and pretty much everything else.