I am sorry to read your news about your fiance. You have already had visa advice - not my area of expertise - and reassurance about his treatment.
The commonest cause of aortic stenosis in UK is scarring with calcium deposits causing the valve to be more rigid. This increases with age ( you don't mention his age ). It gives the heart more work to do, especially if the valve can't fully close in addition, so it can fail, and give rise to chest pains / breathlessness. The valve may need to be surgically replaced by a mechanical valve, or ( if he is not fit enough ) made wider without heart surgery.
You say he's on a heart monitor - this may be an ECG ( electrocardiogram ) tracing, other standard ways of checking on his heart function, or ( if he had it before admission to hospital ) a pacemaker - which is used for various heart conditions.
If blood flow to the brain is reduced because of valve disease, then fainting could result due to lack of oxygen. You mention low blood sugar, which suggests diabetes in addition, although there are other possible causes ( unrelated to valve disease ).
Of course you can search on the internet for any medical conditions, but I've tried to explain your fiance's situation as simply as possible. Unfortunately you haven't given much detail - maybe that's all you know. I would expect that members of his own family, or others closely involved in his care here, would be fully informed by the doctors and nurses looking after him. If he is unable to give you more details, then they may be able to do so. You would certainly need more information before attempting to visit him in the UK.
