The Gemara in Kidushin (32a) concludes that the obligation of Kibud Av is from the father's money. However, Tosefos brings that if the father does not have, and the son does, the son must then support the father.

There is a difference of opinion amongst the Poskim (See Yore De'ah 240) if this is merely a responsibility of Tzedakah, or if when the son is solvent and the father is not, the obligation of Kibud Av extends to include monetary obligations.

For further discussion on the many complex laws of Kibud Av, allow me to suggest the wonderful work "The Fifth Commandment" by Rabbi Moshe Lieber, Artscroll Publications.