Yep... don't plant him outside yet, that might just make things worse since variable conditions are hard enough on established plants. Stick him back in a small well-draining pot and wait. Too much love will definitely kill it, at this point you need to just treat it as if you would an overwinter and wait for new growth to appear... applying anything more will probably just make matters worse since we don't know what caused its original decline.
For a three year old plant the stem and roots look pretty sad, which suggests suboptimal conditions that probably would have promoted disease onset. Happy plants can usually recover from most issues, but there are some nasty things floating around that pretty much guarantee the death of a plant. If it dies, it dies, these things happen. Annums can be be finicky.