Put yourself in his colleagues' shoes panda: he's got a job (presumably the department needed his input) and then went off sick for 3 months leaving a huge hole in the workforce and no doubt putting extra strain on the remaining staff.
So he's proven himself to be unreliable, and he's also admitted to suffering from hypertension, so their might be further reliability issues.
It doesn't sound like they're undermining his work, just showing the fact that their patience is wearing a little thin, and if he'd not spent 3 months sitting at home*, he'd probably be up to speed with procedures by now.
I'm not saying they're right, but there's 2 sides to every situation, and it might help for him to recognise this and work at rebuilding his reputation within the department rather than whining about bullying!
* Right or wrong, this will be the perception of his workmates...