FIL is aggressive when we try to assist with shower and toileting. Any advice?

Great answer from eyerishlass. FIL may be able to wash his genital area at the front, and you can wash the rear without it being such a personal intrusion.

Most elderly people hate taking a shower. They're afraid they're going to fall, they're cold, and they probably feel vulnerable being naked in front of a loved one. Do you think this is how your FIL feels? Does he have shower stool in the shower? If so, before his next shower gather together 3 towels. As your FIL sits on the shower stool keep his upper body covered with a towel. Running through the dryer first so it's nice and toasty. As your FIL is covered on top wash his legs and feet (I'm assuming you have a detachable shower head). Wash in the folds of his thighs and between his legs. With the towel still around him wash his chest and face. Yes, the towel will get a little wet at this point but you'll throw it in the dryer after the shower. Once everything has been washed but your FIL's face take a clean cloth and wash his face. Remove the towel from around him, shampoo him in 10 seconds. Grab the 2nd towel, the dry towel, and wrap it around his shoulders. Take the 3rd towel and place it over his thighs. Once he warms up you can help him get dressed.

The trick is to keep your FIL as covered as possible without creating a mess. It can be done. When I worked in hospice this is how I showered my patients if they were ambulatory. Having a towel on helps keep someone warm and helps them feel less vulnerable. When I'd help someone to the commode I'd throw a towel over their lap as well for the sake of modesty.

It takes a little practice to get the hang of it with all the towels but your FIL will appreciate it.