There is the senior's centre on Stanstead Road http://www.seniorslewisham.org.uk/index.html
However, many recently retired people may not put themselves in the same category as those who may have retired 20 or 30 years ago. But it is worth going along to see if it is a group of people that he gets along with.

My mother is involved in University of the Third Age, which provide a range of learning (and teaching) opportunities. There are a number of local choirs who are always looking for men who can hold a note (and most can) as they tend to be female dominated.

The Forest Hill Society (which I'm involved in) caters for all age groups, and we welcome members and activists of all ages. A bit of planting round the station every so often, trips beyond SE23, walks, newsletter delivery (a chance for a little walk around the area) and the committees to discuss planning, transport, and environmental issues. We welcome people of all ages and there are a few recently retired individuals who are particularly active in the Society as they have both time and energy.

Finally, can I recommend keeping recently retired people busy with grandparenting duties