That could be it, if he can see her he may be frustrated that he cannot get to her. Perhaps if you are not planning to breed them you should somehow prevent them from seeing each other? See if the nipping stops. It may not be the female for sure, it is good you are buying some more plant, it couldn't hurt, perhaps he won't mind the female if he has more activities to distract him.
Did you put the medication in his cup or in the tank? You probably shouldn't add it into the water the female is in unless she needs it as well. But I assume she probably got some nips from breeding and stuff so it'll help those. If you added it right to the cup you should watch him because it is hard to administer doses in such a small volume of water.
Salt baths should usually only be about 5 minutes long at a time assuming you used one teaspoon per gallon of water. If you only used a dash or so it should be fine in the tank until the next water change, but it is safer to do dips rather than leaving salt in his tank water.
This is because they can have troubles balancing their water and salts in their bodies when placed in salt water...well because they are fresh water species as you know. For tail biting you should only have to do one or two and he should heal up fine. Mine just had one to prevent infection and he healed up pretty fast
Good luck with him, I am sure he will be fine once you figure out what he is fretting about. Generally the nipping is much easier to heal then finrot and stuff which is good
You are doing a good job