Cat Behaving Unpredictably After Illness

In May, my moggy had a severe bout of anorexia and wouldn't eat for almost 3 weeks. He went to the vet 5 times and was hospitalised for a week. He was also diagnosed with IBD during his hospitalisation. While he was sick, he bit the vet and me, which was really out of character for him. He also growled at the hospital staff. I'd never heard him growl before he got sick.
He's recovered well now, but 2 weeks ago, when I was having a housewarming, he suddenly bit my friend's 2 year old son who was standing near him. The next day, he bit a family member who was playing with him. Tonight, he attacked my foot several times. It wasn't serious, there were no visible bite or scratch marks, but again, this is very out of character for him. How can I help him get back to being his old sweet self?

It could be that he still isn’t feeling 100% or it could that he is still annoyed over the vet visit. Many many cats make it known that they hate visiting the vet through growling and biting.

In any case, first thing to do is to protect visitors. When someone is over I would shut him in another room and don’t allow him out. Maybe at some point in the future you will be able to again, but not until he settles down.

As far as attacking your foot, I think he was probably feeling you were too close or might scoop him upand haul him to the vet again. It is just a sign of his continued anxiety. You can try a hiss or scolding him but there is potential will make him more anxious so I would try talking to him softly in a soothing voice to see if that calms him.

It may take weeks before he begins to feel secure again but barring any new vet visits or illnesses he will return to his old self eventually.

If his behavior went back to normal and now he has started to become aggressive again, I would take him back to the vet. It would seem like something is bothering him again.

Does the biting come with any warning noises? Are there any other changes in his eating or bathroom habits?

He may be in pain, or have an infection. Is he eating, drinking, pooping and peeing? I would take him back to your vet or perhaps get a second opinion and go to a specialist. If he never bit you before, he is in pain, and something is wrong, IMO.

In addition to all of the above....have his bites drawn blood? or are they just painful? There is a very importance difference between the two.....one is distressing, and the other is outright dangerous for both the victim and your boy.

One can be a sign of pain, the other of something that indicates more serious illness and/or requires more serious medication.

[Edit] From what you described....
Young children can be a bit loud and random and energetic. Reactions of anxious cats in proximity to them are....unpredictable. If no blood was drawn, I wouldn't stress too much (and also, try to explain this to your friends!!!)

Your other family member was playing with him, so possibly your boy became overstimulated? Or maybe have moved in a bad way that caused him pain, and he took it out on the nearest person.

Mostly he's calm and placid, but once in a while, out of the blue, there are incidents like this. The bite on my friend's son drew blood, and it got infected, but thankfully it healed. When he "attacked" my foot, there was no blood, bite marks or scratches. His eating, peeing and pooping are all normal except that he vomits a bit (sometimes food, sometimes just water, sometimes bile) everyday in the early hours of the morning, but that's to be expected since he was just diagnosed with IBD.