Corona and fasting tisha be'av

If a person has been hospitalized and treated with oxygene (no respirator) for coronavirus several months ago and has still some symptoms today (no sens of smell, fatigue, etc.) should he fast on tisha be’av ? and if he should, should he use the technic to drink or eat small shiurim ?

To answer your second question first, there is no benefit to eating smaller shirum on 9 av. That’s only on Yom Kippur. The reason is, someone who must eat is patur from the fast, Chazal did not institute the fast for him.
Regarding your first question, I believe you need to ask (a frum) doctor whether he should fast or not. Will his health suffer if he fasts?

Additionally, Can the fellow make it through to the end or will he likely have to break his fast early, If so he should just fast at night and not the next day.