"His brother is living there now and pretending to be noble by taking care of the grampa but his real reason is to put him in a home."
So your bf needs to go out there frequently to check up on grandpa, because his brother does not want to have to take care of their grandfather. Many men are like this, while women are not, so realize this is not unusual.
"... but grampa is still custodian."
Does that mean grandpa has the right to live on the property until he dies ?
And he has the right to make improvements or other changes to the property - but no right to take out loans or liens on the property ?
Here is my suggestion - go with your bf to the property and make a video of your bf and his grandfather discussing what grandpa wants to do for the next few years. The key is to get grandpa's wishes recorded so if your bf's brother tries to do things against their grandpa's wishes your bf can go to court and get a cease and desist order.
For example, why is there a need for permits for the houses?
Were they built or remodeled without permits and are within a city limits?
Taking care of an elderly person whose health is declining can be an all-day job and it will wear out even young strong adults, and it can also be discouraging to a caretaker.
There is no easy solution to this situation, especially if the brother and mother are scheming to get the grandpa off the property.