Im on a JTWROS deed with my father.
Father has been found has been found incompetent and placed in a nursing home.
Can I add my son to the JTWROS deed ,without any repercussions ?
My father is not on Medicaid as of yet.

If he has been declared incompetent, then a guardian has been assigned for him. Whoever that is would have to sign off on any change in the deed.

You aren't being clear whether you want to remove the father replacing your son, or just add your son. Also, if you are trying to do this to avoid Medicaid going after the property, they will look back 5 years (I think) for changes in ownership to avoid Medicaid issues.

I will agree, also the issue of who and how the nursing home is being paid for. If you are trying to protect the property from attachment by the nursing home, They do look back at any "sale" of property that could be determined to hide assets.

I would not do this without a real estate attorney to guide you.

... Can I add my son to the JTWROS deed ... ?

You wouldn't "add" the son to "the deed". You would add him to the title by executing a new deed. I assume that what you want to to is to convey your contingent interest in the property (your expectancy of getting the fee simple absolute after your father dies). You will own the property only if you survive your father; it is possible that he might survive you, in which case your son would get nothing (except perhaps by inheritance from your father).

If, on the other hand, what you are suggesting is somehow supplanting your father's interest in the property with your son, no, you cannot. Your father would have to assent to that and, as you indicate, he is legally incompetent, he would be incapable of doing so.

Also, fiddling with a JTWROS may "break" the JTWROS, turning it into a tenancy-in-common. Check with a real estate attorney in your jurisdiction.

As title to the property is currently established, after 5 years and before your father dies, Medicaid could, I suppose, bring an action to partition the property, sell it, and pay you for your half interest. Asuming this doesn't happen, you should wait until he dies to convey it to you and your son by a new JTWROS deed.