Definitely orienteering! Me and my family are absolute map geeks. DD and I both do a horse riding navigating sport, but DS and DH also get in on the orienteering front. We go as a family on a weekend through the winter and then after we have run through a forest with a compass and map we feel we have earned our Sunday lunch (which we usually have at the pub.) We’ve found orienteers to be a very friendly and encouraging crowd, who are always keen to help kids get started in it. We only took it up in order DD and I to practice our compass reading skills for our horse sport, but we’ve got so into it that we’ve been on our club teams at national level events, and the DCs were on our club’s Yvette Baker-winning junior team (this is a junior team championship event.)
Other mappy things he can do include geocaching (although this is a bit more limited in its map reading skills) and map puzzles. The Ordnance Survey do quite a good puzzle book which is excellent for getting non-map readers familiar with OS maps. I also have the OS app on my phone, and you can use it to map read when you are out walking. Get him used to reading OS 1:25k Explorer maps - buy a paper map of your local area, get a compass (not a cheap rubbish Amazon one, it will frustrate him with its not-working-ness! Silva do a decent one for about £15 and the Decathlon ones for a tenner are good too) and learn the method to read a bearing from a map. Scouts teach it and call it ‘red in the shed,’ and you will also find details on it on the Silva website under the ‘Silva 1-2-3’ method. It is a bit complex for an 8 year old, but not incomprehensible for him. My DCs were both capable of taking a bearing from about the age of 10, but loved having a compass anyway. once he has got compass readings sussed, teach him how to find grid references on OS maps. There are plenty of tutorials for this online.
Also buy him a whistle (I know, I know! 😬) and teach him the International distress signal on it - not SOS in morse, although that is useful, but 6 long blasts. Teach him that it is only to be used for real emergencies.
Finally, check out your local Cubs group, and see if they do a lot of outdoor activities. They tend to introduce basis hiking and map reading skills, but don’t really start to use them till about 12 (in Scouts.) But there is a lot of fun to be had at Cubs wrt outdoor activities.