What do we need to know re: the financial situation?
Right now his pension and SS goes directly to the care center.
We and his son, (our uncle) are his only family, and all are on board and believe it will be the best for him.
I (his grandaughter-in-law) have been a care giver for others over the last 3 years.
Financially the money will no longer be going to the care center, of course. Was anything else pledged to them? Will there be a final bill of some kind? Read and understand his contract so you know what to expect on that end.
Have a care agreement drawn up stating what services you provide and what he is paying -- and/or a rental agreement for room and board. Since his entire family is on board it may not seem to matter whether you have this in writing, but if in the future Grandfather is sicker than you can deal with and needs a nursing home, in the application process for Medicaid this financial transactions for the last 5 years will be examined. It is perfectly fine and expected that he use his money for his care and room and board ... but it clearly needs to look like what it is, and not a gift to you.
I hope this really does turn out to be the best for him, and not too overwhelming for you. Remember that if conditions change your decision can change. Please do not promise him that he will "never go to a nursing home." Instead promise that you will always see that he gets the care he needs.