A boarding house in CA has to have a license and is required to provide a safe and secure environment. They need not have private bathrooms but the bedrooms can not house more than 2 persons who are un-related (aka un married) unless the boarding house is also registered as a SRO [single room occupancy - where the clients share the kitchen, living room, and common areas]
The big red flag is that fact that your BF has to go outside the sleeping area to the outside of the house, around the corner and into the house to get to the bathroom. If they had a hallway he could use to access the bathroom, then it might be more legit sounding.
I would also be concerned about the legality of the garage In-fill (In-Fills are garages or out buildings that are converted into bedrooms to bring income to the homeowner. Many times they are illegal because the property owner did not get the proper exemption or certification!)
You can ask for the certification of the infill. If they can not produce it, then walk away. If your bf has already put down a deposit, then request it back in full, otherwise you will challenge the owner in court. FYI: In CA the loser of the court case has to pay the legal fees. Small claims court is a mere $90...which may be well worth it if he has put down something like the usual here in LA of $450 or more!
Any place where you must exit the room then enter the house to use the bathroom facilities is in VIOLATION of the State boarding house regulations.
The basic difference between a boarding house and a rental of a room in a house is the license. A boarding house has to follow many more rules/regulations about safety for the residents.
If your boyfriend thinks that, the situation is acceptable to him...that certainly is his choice. HOWEVER, be aware that many converted garages are not heated or cooled.
Stay away from those sort of places...there are many places where you can get several roommates to rent a large place. Then share the utilities!