Hi Mukta!
Sorry to hear things aren't good! Illness can really mess things up!
I don't know if I have tha much advice on what to do really. My DD hasn't had a night feed since we gave up DF at 10.5 months, and that's not really a night feed anyway... so even though she can wake in the middle of the night, she's never expecting a feed. If DS is genuinely hungry after not having an appetite he'll obviously need the extra calories, so problem is how to get him to have them during the day instead of waking at night for them. I'm sure you're already trying your best to get more into him in the day, so hard to advise really. Maybe letting him have more of his favourite foods? Adding extra fat e.g. butter or oil to his food? I was told you can add it in almost anything, porridge, fruit puree or yoghurt, or in his dinner foods. Where is he at weight wise for his age/height?
Otherwise maybe it's a matter of giving in to the night feed for a short while and trying to up calories in the day, and then if he doesn't stop waking for the milk in the night you might have to go back to sleep training again, WI/WO or PU/PD... unfortunately...I know that's hard work but it will probably be necesary if he doesn't all of a sudden go back to sleeping through on his own.
I'm only thinking what I would do if I was in your situation...
Also, if he's been unwell some of it might be that he's OT too, and less able to settle himself when he wakes cos of that.
I'm happy to share my thoughts and bounce ideas, but I'm no expert on the subject hun. Hope things settle down quickly!
Lots of hugs to you!!! xxxxxxxxxxxxxxxxxxxxxx