Martin: In this context, Фёдор почувствовал неладное is correct, because the meaning is basically "Fyodor sensed something-not-okay." The neuter accusative adjective неладное ("a not-okay thing") is functioning as the direct object of the verb почувствовал.

On the other hand, if you wanted to say "Fyodor felt not-okay", you would use the construction Фёдор почувствовал себя неладно -- note that неладно is acting adverbially, and so it doesn't have to be in the masculine short form (неладен) to agree with Фёдор.

But using that почувствовать себя construction implies that the feeling is internal (i.e., Fyodor was sick or something). In this context, though, Fyodor senses that something external to him is amiss -- he "feels suspicious of something" -- and that's why Paul recommended the construction without себя. Hope this is all clear.

Also, as a general note, остановить means "stop" in the rather specific sense of "to halt someone's or something's movement/travel" -- as in Милиционер остановил велосипедиста, "The policeman stopped the bicyclist." And you can simply put a -ся on it to mean "to halt one's own motion" -- Автобус остановился, "The bus stopped."

But if you mean "stop" in the sense of "to cease and desist from the process of doing something" (such as eating), then the verbs that Paul recommended (прекратить or перестать) should be used instead.

P.S. By the way, your Russian is definitely improving -- keep up the good work!