Try a large supermarket (Tesco, I think I've seen them) - there's usually a section near the "feminine hygiene" with Tena pads - assorted thicknesses / absorbencies.
I'm sure I've seen ones for blokes next to the women's ones, but not being of the male persuasion, I haven't investigated.

At the age in question, tho', he should be able to get the necessary, delivered, via the NHS. see his GP.

Hope this helps
Paws