Your best long term fix is to replace the steel section which has corroded. If you can find a mobile metal guy (hello Craigslist?) you can temporarily support the post, cut off the bad section, and weld a new piece in place. The trick will be the attachment to the bolt in the cement.
You can do it in reverse. Chip out the cement, remove the rusted section and the hold-down bolt. Fabricate a new section with a threaded piece on the inside. Thread the new hold down bolt into the new post section, leaving a length that will be buried in the cement. Weld the new section so its bottom is level with where the cement will be. Pour the cement to bury the bolt and level it to the bottom of the new section.
Prime and paint!