He's a geek. He taught us programming when we were young and loves tinkering. Hes also horrendous to buy for at Christmas. Would a raspberry pi be something he would enjoy or are they just aimed at kids?
do a search in geeky_stuff for Raspberry Pi too - the questions have been posed here before...
I think it would need some TV with HDMI input plus PSU and USB keyboard and mouse, for starters.
Whether he'd enjoy tinkering with it is another matter - even though I've 35 years in IT, I'd find it difficult to devote much time (while growing a business too) to learning to write software in C++ or Python or similar.
Many users will be using an 'off the shelf (internet)' download, either for something specific, like making the unit act as a video/ audio player, or will be wanting to write code for specific purposes.
It's a general purpose system, from what I've read, but more 'bare bones' than any home computer that one bought in the 80s and early 90s.
While looking for something else I noticed a "Raspberry Jam" meeting in Manchester (Eccles) on Saturday. It's the sort of thing you might consider visiting with your dad to see if there's any interest.
This particular meeting is aimed at Raspberry owners as part of a 2 day event on computing and the curriculum, with a full day tomorrow called 'Hack to the Future' teaching up to 150 pupils from 0830 to 1500, then an adults only debrief over tea and biscuits (no doubt for teachers to discuss the teaching content) and in the evening 6-9 'Family Hack Jam' and then the Saturday event as already mentioned.
I know it'd be something of a giveaway about an idea for his present, but better to find out if there's interest first than no interest, afterwards, eh?