You should receive 100% of his benefit — minus YOUR deductions for Medicare. You can find out exactly what he was getting (the base monthly amount) on his account at Medicare.gov (if it is still an open account after his death) — OR by looking over last year’s tax return you filed including the Medicare gross amount on the form they sent him. (That’s assuming he died last year.) Or the previous year, if he passed mid-year. Then divide by 12 to get the monthly amount before deductions. That’s what you should be receiving.
And this year, you should get a bit more because of the 5.9% inflation COLA.
While the people who answer questions online at SS are often dumb and rude, the software typically gets it exactly right. So if your first payment on his account doesn’t seem correct, please write back.