Ok, based on your additional messages, OP, there are two issues, and I will say that having to go on long-term illness leave for 4-5 different issues in 4-5 years definitely puts him in a very small group of people with truly rotten luck. The only people I know who have a work pattern like that have a chronic illness and have had to go off on long-term illness leave more than once to manage the same condition, or one person that got into an auto accident and then had lasting effects, so I would have considered that a chronic illness, too. I suppose that because I DO happen to know some people with incredibly rotten health luck, I just take things at face value, and would assume this man has had a tough time of it, but I do understand two things: 1. that statistically, sure, he’s outside the norm, so you’ve going to wonder occasionally if he’s taking the piss. But 2. This is mostly about how your bosses have treated you.
I think your feelings about whether he’s taking the piss or not, though, are affected by #2 above, the same thing we see time and time again on here: a corporation just forces an OP and their team to cover for someone on long-term sickness leave without any additional help. At worst, we’ve heard from OPs that are left doing a colleague’s entire job as well as their own (harder to be sympathetic when you have to do someone else’s whole job, no matter what they’re ill with) or, like this OP, a department that is short one individual so everyone must work together to do that person’s job.
Corporations are the bad guys in these scenarios, and I’m sorry they’ve done it to you, OP. I imagine Xmas luncheon would be much different if you could have a pleasant conversation about how much you like the temp worker filling in for him during his illness, versus quietly seething about how much work he’s added to your load. Blame your bosses.