Help. I know I must get a will, power of attorney, and healthcare proxy done, but my son has mental illness. Any suggestions?

See an elder law attorney right away. Your son will need any assets you have placed in a special needs trust. You should not be attempting now to live with him or have him in your care. You need all the help you can get now to comb this out. Take the person you "have in mind" to the lawyer with you to help with questions and answers. Do your own directive for health care, interview hospices. It may be time now to go into assisted living and to spend up your own assets on your own quality end of life care. Your son, if he is ill enough, may do well now going into guardianship of the state if he is severe enough. So sorry you are going through all of this. I cannot even begin to imagine how hard this is.

I would say that you need to care for yourself first so..
Talk to an Elder Care Attorney when having the documents done. You can ask a trusted friend to be Health Care Proxy, POA and if they wish Executor of your will. I would think about establishing a Trust that will help manage funds to be used for your care.
If you wish maybe the Trust could also have your son named so he becomes a recipient of any money and that could be used for his care Like a Special Needs Trust.. (not sure he would do well if he were just given the funds from your estate. If there is anything left)
This really is something that an Elder Care Attorney should handle.