You are obviously a concerned neighbour
If only there were more, vulnerable people would be a lot safer. However, your knowledge of his circumstances is very patchy and I don't think you are going to be able to get to the bottom of it and then give the help needed or wanted :?
Often people are taken to a busy A&E, get patched up and discharged, no one really digs deep enough to find out what the underlying problems are.
If he is driving and sometimes lapsing in to unconsciousness the serious safety issues trump his employment needs. And if he is working on a farm - there are lots of serious safety concerns in working with chemicals, heavy machinery and large and potentially dangerous animals.
You are clearly very concerned and want to help, but I don't think you can offer the help that he needs. Someone who is in this kind of state usually needs a lot more than a cookery lesson and access to a microwave. The sister is struggling already from what you say.
If he is working he is clearly not in decent/adequate housing. Could he be being exploited by a landlord or by an employer? I don't know what you mean by "learning difficulties". From what you say there are also issues for his sister/carer who is struggling to cope. This chap is struggling to look after himself and there seem to be issues of neglect. You paint a picture of a fairly worrying situation.
I think you need to make contact with your local social services department and say you are very concerned about a "vulnerable adult" and be very clear about why you are worried, don't hold back on your worries - people die in hypos. They will have to investigate and get to grips with the things you are worried about and they have the power to do this properly. They may even know the person concerned already.
I think this needs a proper assessment that will look at any concerns around exploitation, neglect and safety and the support the sister needs to look after her brother. Asking social services to find out what is going on could help them to find decent housing, claim benefits if needed and get him, and his sister, the help they need to live independently.
There is a lot of support available and it is usually provided in a very discrete way. Social services are in the best place to get this ball rolling after you have talked to them.