One more update.
2 weeks ago I decided to start treating him with anti fungal medicine and a little bit of salt, as his body was still with the white spot and his fins didn look good.
After finishing his treatment, I changed his water 75% daily, with a little bit of salt diluted, for a week.
Now, Nebbie is much more active and going around up and down in his aquarium, his fins open and full while going around greeting us when he sees us. His scales are not looking great yet, but his overal behaviour tells me he’s going to survive!