This obsession with sex is known as Inappropriate Sexual Behavior or ISB in the world of dementia. There ARE medications available to calm your husband down so please speak to his doctor about the situation immediately.
Here is a link to a FTD dementia chat support forum you can sign up for:
https://www.ftdsupportforum.com/
If you Google "FTD dementia chat" you'll get a lot of useful support info to come up.
Get him some sex toys and let him pleasure himself, or, if you are so inclined, help him use them! Just because he is old, doesn't mean his desires are gone.
Here is a link to a FTD dementia chat support forum you can sign up for:
https://www.ftdsupportforum.com/
If you Google "FTD dementia chat" you'll get a lot of useful support info to come up.
Best of luck.