For prostate cancer to be causing noticeable symptoms there would need to be a lot of cancer cells, and a lot of cancer cells would cause a PSA of perhaps 50 or more. Now there are a few very rare variations on prostate cancer which could cause symptoms with a low PSA, but these are so rare you need not worry about them.
There is a small chance the cancer has spread, that is always possible, but with a PSA that low they will be micro mets and not causing any pain.
So at the moment, I would say there is no evidence of spread and your husband's leg pains are not caused by cancer.