My husband and I ( now in our 70’s) have done done active, hiking-focused tours in small groups to Patagonia, the Alps (Tour du Mont Blanc), the Spanish Pyrenees, Slovenia, and Japan with REI Adventures or MT Sobek ( a Berkeley-based company). The group size has ranged from 7 to 10 people, mostly in their late 50’s or 60’s, of which at least 3 on every trip have been solo travelers. Often the solo person is married, but their spouse could not, or would not for some reason, join such an active trip. On the Japan trip, which was 7 people plus our guide,, we were the only couple. The other 5 were two men whose wives did not want to come along, a woman whose partner had already done the trip and could not get time off work, and two widows traveling solo for the first time. Each had something positive to offer the group and we all enjoyed each others’ company.
One of the highlights of almost every trip has been when we reach an alpine lake on a day’s hike, and my husband strips down to his swim shorts and dives in for a swim. The guides are amused, but in most cases other men, and sometimes a woman or two will join him ( not me—-I don’t like such cold water!
REI no longer does foreign trips, but I would encourage your brother to look into the Dolomites or other trips offered by MT Sobek. They make it easy—- the meeting place is usually a major airport or a hotel in an easy-to-reach city. All one has to do is show up with the right gear, a good level of fitness, and the readiness to have a great time. Cultural exploration as well as good food and drink are part of the whole experience.
HF Holidays, mentioned above, would be another great option. They offer hiking-focused holidays from their own country houses scattered across the UK, as well as week-long holidays in good locations around Europe (Austrian Alps; Tour du Mont Blanc; Dolomites, Puglia and other locales in Italy; Costa Brava in Spain, and much more). All generally offer daily guided walking on 3 levels. (easy, moderate, and strenuous) and lots of resources for further exploration on one’s own. We have enjoyed walking holidays with them in the Lake District and Jurassic Coast of England. He might like to consider their Scotland base in the Highlands, which includes an ascent of Ben Nevis for those who are interested.