Aquarium salt is different from table salt in that it does not contain iodine or other anti-caking agents. If you can't find aquarium salt, yes, you can use a tsp of table salt for now but AQ salt is preferable just to be on the safe side.
Neil has a point that for his regular home, you might want to consider something larger for Seb's home so your 10L tank would be perfect. However, it's okay to keep Seb in his teapot if you don't mind changing his water every day or every other day at the least. I suspect that Seb's white spots and his other symptoms are due to ammonia poisoning, which is caused by not enough water changes. Fish excrete ammonia through their gills as part of their waste but ammonia is highly toxic to them. And even though the water LOOKS clean, the ammonia levels can build up very high and very fast. Don't worry, it's an easy mistake but also easy to fix by upping the frequency of water changes. However, fixing Seb's symptoms might be a little more difficult.
If you'd like, you can leave him in his teapot for his treatment, I didn't realize it was so small. He may feel more comfortable in his familiar home. Just be sure to lower the water levels for him so he can easily reach the surface. He has dropsy, which is a symptom of ammonia poisoning. Dropsy causes fluids to build up in his organs, bloating him up and forcing his scales outward in a "pinecone" effect. If he reaches this stage, it can be very hard to reverse. :( Dropsy also causes his organs to push against his swim bladder which is why he has having such trouble swimming and his tail is dragging. Because of the severity of Seb's symptoms, I would suggest upping the dosage of salt to 2 teaspoons of salt per 4 liters (I'll switch over so it's easier for you to keep track
). If you don't have any way of heating his container or teapot, try wrapping a towel around it so it can stay warmer.
Continue to fast him and change his water every day, redoing the salt treatment each time, for ten days. The best thing for him is clean water.
If however, he looks like he is getting worse and seems to be in distress, post immediately. He might need to be euthanized. :( But we'll do everything we can to avoid that. Hang in there and keep us posted.