I wrote ''he felt okay'' and is supposed to be wrong.
I think that phrase is ok.
It depends on the context. If something external made him to feel something, you can drop "se":
— A crise econômica está afetando vários comerciantes. O João também sentiu o efeito dessa crise em seu mercado.
— Sim, ele sentiu bem. (that is, he could notice the impact crisis had on his business).