How awful!
My boss has been tremendously supportive of all my health issues. Horrible that you've got such an a*******e of a manager.
Keep a diary (this was useful for me when I ended up reporting a previous manager (previous company) to HR for reasons unrelated to my health/this topic) I was successful
Check your HR policy re harrassment/bullying.
He's going to have tread very carefully if he thinks he's going to manage you out because of you being ill.
Many companies do have a 'capabilty procedure' where certain triggers warrant a formal review e.g patterns of short sickness absences - don't be afraid of this, it's usually a routine thing, I hate when managers don't re-assure their staff of this procedure and often staff get very worried that they've done something bad by being ill and absent from work (I used to be a manager by the way and would like think I was considerate to people) - at the end of the day a company needs people to work, and their approach of doing a formal review is to identify if the person needs additonal support or if they are ultimately capable of doing their job because of their illness.
What was the outcome of the review? Where there objectives discussed, support plans, change of your duties, shifts etc?
The Occupational Health Assessment get involved to help improve your working environment - it should be from a company care perspective. Personally, I've not had to have this for my Diabetes but it's not affected my attendance or ability to work (just the appointments I have to attend) I did have one because of my spinal condition once I got diagnosed, I had an assessment and I got a special chair and my manager was advised I was allowed to take microbreaks because sitting too long causes compression of my spinal discs and excerbates my nerve problem.
Ultimately you need to consult HR and not your boss on this matter. Find out if it's standard practice to involve Occ Assessment following a formal attendance review. Also if it's to 'assess your Diabetes' what outcome are they looking for here? Will they give you more breaks, change your shifts, allow extra time for taking medications...I actually don't understand it to be honest. I do hope it's not going to turn into you having to report how much/what you eat, how much excercise you do etc, that's incredibly personal and intrusive...if someone's had any other illness say IBS, do they have to report how often they go to the loo and how much fibre they've had???
Sorry I can;t be of more help, but just wanted to re-assure you someone here cares.
Good luck