It might help to know what age you're talking about. I don't know that our erections get smaller but I suspect that because of that being controlled by blood flow that that could then impact rigidity and possibly then size. Yep, be very careful about discussing this with him without proper information, we guys are pretty locked into being OK about the size of our penis!
Usually a penis will not "shrink" for any reason (age included) but as others have mentioned above, 2 things could possibly be going on:
a) For some reason there is less blood flowing to his penis, causing it to not reach it's former capacity,
b) There may be some form of Peyronie's disease going on which is causing his penis to become curved or constricted in some way.
Reasons for A) above can be psychological, health reasons (is he overweight or suffering from diabetes?), or due to prescription / non-prescription drug use.
The best thing to do would be to try to discuss this with him first and find out if he is aware of it. I'm not sure what to tell you as far as how to approach it, but I suppose it would help to just reassure him first that you love and care for him and the relationship and then ease into the discussion of the matter. But NOT discussing it is probably going to keep the problem going since he doesn't apprear to realize that there even is a problem. Good luck.