My sons mom and I have been divorced 25 years. We did not remain close. My son is 30 and married. His mom is dying. Am I obligated to go see her dying?

There is no legal requirement, and honestly no real moral requirement after all of these years.

If you son would like you to, and the doctors do not believe it would upset her too much I would for the sake of the living ( your son)

You don't have to, that choice is yours, but even if you don't see her while she's living, it'd be nice for you to attend the funeral in support of your son.