Contrary to popular belief it is possible to warn or dismiss someone for unacceptable sickness levels (on the grounds of incapacity) - even with doctor's notes - but it isn't necessarily easy! You need to be ensure that anything you say or do is both reasonable and justifiable as well as ensuring that you stay the right side of the Dosability Discrimination Act (DDA).
Because it is so easy for an employer to slip up, I would urge you to get some advice obout this from an employment law/ HR specialist before you do anything.
You don't necessarily need anything in your employment contracts about levels of sickness, although it helps to have a policy. You haven't said whether you offer paid sick leave or whether the employee receives SSP only - this is the sort of thing that would go into the employment contract. However, if you haven't issued your employee with an up to date Statement of Terms and Conditions of Employment (commonly referred to as a Contract of Employment) AND if you haven't got a Disciplinary and an Employee Grievance Procedure, then my advice would be to get this sorted out before you take any steps (I'd advise you to get these sorted as a bare minimum anyway, even if you decide not to take any action at this point).
From what you've written, the levels of sickness would be classed as unacceptable from the perspective of most employers. Many employers routinely do "Return to Work Interviews" as part of their sickness absence management procedures, and it is perfectly acceptable to conduct these, but you need to be aware of the best type of approach to take, particularly if you are intending to introduce such a procedure now.
There are some hoops you need to jump through to ensure fairness (from a legal perspecive), and the employee needs to know that the sickness levels aren't acceptable and be given an opportunity to improve. Most employers would follow a procedure that has an informal stage and a few stages involving formal warnings - with dismissal being the final step when things don't improve or the employee is too ill to work. The procedure would also allow for an appeal against warnings and dismissal. Employers also commonly ask for the employee's permission to approach the employee's GP for a medical opinion (the employee is entitled to refuse). Many employers also get their own opinion from an Occupational Health specialist if necessary. Sometimes the causes of absence are unrelated, but on occasions there is an underlying condition. The OH specialist can also give an opinion as to whether the employee is likely to be covered by the DDA and/or whether there are any recommended "reasonable adjustments" to the job or working conditions.
This may sound like a slog, but the alternatives (doing nothing, or jumping in and warning/sacking the employee unfairly) can lead you into such a tangled mess that it's worth taking the time to get it right from the outset, hopefully with an end result that gives you back your productive employee.