Gifted house to son without life tenancy. What to do now?

It's possible that if there was an "implied understanding" or "implied agreement" between you and your son that he would continue to allow you to live in the house for life, at the time you deeded the house to him outright and without any reservation of a life estate, that you now have a "constructive trust" interest in the house for life.

As you can see, this is a very technical legalistic point that only an expert attorney in real estate law can help you with. Thus, I suggest you consult such an attorney for advice about your situation. Hopefully you will be able to stay in your former home.

is the house deeded directly to him? are you still living in it?

You need to talk to an attorney. One wonders why you put your son's name on the deed.

DebraJo, I don't see why you would have to move, unless you needed a nursing home. You will not be eligible for Medicaid, because you gave away a very large asset. Medicaid looks back 5 years into your financial history and if the house was worth $50,0000, well, they will not pay for the first $50,000 for your care, if you gave it away within 5 years.
If you did this more than five years ago, you should be OK.

Can you not "undo" this?

Don't sign anything else until you see a lawyer and whatever you do, don't move out.