How can one anticipate when someone will be feeling bad at a particular point? Or is this like the colloquial meaning in English (He must be feeling bad because he's done something wrong)?
In either case a little context would be helpful: "Your son will be feeling bad because he broke your favourite cup." Or "Your son will be feeling bad tomorrow after eating that soup."
Besides, a more natural wording in English might be "Your son won't be feeling well", but again it depends on context.
To me it sounds like a curse or a threat. "You dare fire me? Your son won't be feeling so good soon...."
It is much more natural in English to write 'your son will not be feeling well' but I guess you'd write this differently in Polish
Corresponding sentence would be: „twój/wasz syn nie będzie się czuł/czuć dobrze”
Well, "zły" (the adjective) can mean all of those, but I can't think of a situation when I could interpret the adverb "źle" as either 'mad' or 'evil'. Just 'bad', 'unwell'. He will either feel physically bad or mentally bad.