npatrick,
Although you don't state this explicitly, I would guess that your husband is grieving the sudden and dramatic loss of sexual function that so often accompanies radical prostatectomy. For most men, this is a very depressing situation, and in that sense, depression is probably a pretty normal response. When I found out that there was a 50% chance I'd end up impotent from this operation, I started getting depressed even before the operation took place.
I think sending the message "I love you anyway" or "I love you no matter what" can make a man feel even more depressed, because each statement contains the poignent reminder of his loss. It's probably better to just say "I love you", and allow him to grieve. It's only been four weeks. That kind of pain often doesn't resolve itself quickly. It shouldn't be minimized.
I agree with Geifer that it is helpful to talk with others who have gone through the same thing. As a prostate cancer patient, I know that he is having a normal response, but he might not be feeling "normal" right now.