I have no idea what to get my FIL for his birthday! Any ideas?

You can gift him a book, some music CDs with singer of his choice. If he wears specks, you can gift him a nice cover. You can personally make a card for him by checking out many DIY videos on the Internet. You can take him out on a surprise dinner or lunch calling all other family members there in advance. You can get him nice shirt or if he plays sports then can gift him a related item. If he loves pets, you can gift him a cute little puppy. For more check this out: http://www.gifts.com/categories/gifts-for-dad/3hV?sk=&ref=GFTorganicgglunkwn&prid=gftseogu