Phone the hospital (or phone round all of the likely hospitals if you're not sure which one he's in) and ask if he can have visitors. If they say 'Yes' go and see him. (Take him some magazines to read or some other small gift). He'll probably be able to either give you the keys or tell you how you can het hold of them.
If the hospital say that he can't have any visitors, try explaining your situation to them. If he's conscious they might make an exception for you.
If you can't get the keys but you've already signed a tenancy agreement then you've got the right to access the property. Contact the police to let them know what you're doing (so there are no misunderstandings) and then break into the property. The cost of (for example) replacing a broken window and changing a lock is likely to be far less than the cost of staying in a hotel (and storing your furniture in a warehouse) for an unspecified length of time.