It might be a mistake from your employer. I would suggest to talk to him and clarify any confusion. Remind your employer about the letter. As you are on "sick leave", so you are not liable to attend the meeting.
I'm Off Work With Stress/depression, But My Employer Thinks I Have Been On Holiday, And I Haven't, I Have Never Been Sick, Now He Asks For A Meeting To Discuss Terminating My Employment Can He Do This?
During the 10 months that you have worked thee have you missed a lot of days? If its a small company and your work record is poor I would think that if they wanted to relieve you of your position they possibly could. You need to talk to human resources at the company to find out all the details. Good Luck.