He could stop the aging process by dying.
Living is a coherent and ordered process, death is the cessation of that coherent and ordered process.
Ageing is the process by which a living thing accumulates disorder. Once the disorder reaches a certain threshold the system is no longer coherent and ordered, and death occurs.
When an entity dies, by definition the thing which could age, no longer exists. Yes there are its components and sub-components, but the thing itself is no more - and therefore can never age.
Aside from the definitional vagaries, the individual can effectively slow/stop aging by performing biological maintenance and remove the disorder from their systems. If they can remove it faster than it occurs that would be a fountain of youth, otherwise its on par, or a slower aging.
To accomplish this the individual will have to understand the biological computer of each and every cell species in their own body (both the human lineages, and the foreign symbiotic/parasitic lineages), and the biological computer that is the body (which also includes the mind).
Needless to say, this will require an extra-ordinary intellect. There is a reason why the somatic nervous system is in control over our guts, we the non-somatic nervous system simply do not have the intellect to handle it day in day out. While not definitive, I recommend reading Mind: A Unified Theory of Life and Intelligence Kindle Edition
by Frank T. Vertosick for a thought provoking discussion on what life is, and what intelligence is.
Given they have access to a source of all/much human knowledge it is possible that given a specific symptom, they might administer and/or correct certain chemicals in their own/other bodies with some varying degree of success, not unlike modern medicine. Certain chemicals, procedures, alterations, etc... will have known effects, yet might still have knock on effects that won't manifest symptomatically for years.
Given that altering chemical makeups is probably dangerous, but more specifically very fiddly and likely hard to do at scale. One useful case might be in combating Prion based disease.
Prions are chemicals capable of self-replication, and can even refold other similar chemicals into their shape. Many prions are indeed useful biologically, others are highly dangerous. Mad Cow Disease is caused by prions that were misfolded from their biologically useful structure.
A suitably trained individual might be able to disassemble incorrect prions, or refold them into the correct shape. Given that they are self-replicators, these would quickly produce benefits, or at least avoid the issues.
Alternately the cell could be asked to activate a repair pathway that for some reason has been de-activated or isn't activated enough.
The essential proposition is that by assembling certain chemicals within the cell, the cell can change the way it responds. The response being the desired effect of killing the cell, or causing particular repair functions to occur. These response occur because the chemicals being produced and in what quantities change, and perhaps even the segments of read DNA are shifted.
It might (though this is far from proven) even be possible to treat cancers this way, essentially renegotiating the cells role in the body by asking it to adopt this or that role (given that the DNA governing that role is still correct).
Would this stop aging? Probably not, there are many sources of disorder in biological systems. But it might slow, or alleviate certain disorderings, and hence slow aging in those individuals afflicted these ways.
It would probably still be more effective to use medicines engineered in a laboratory/factory. This method is scalable, where trained nano-surgeons (the chemical manipulators) could not scale so readily or easily, even though it would be beneficial for them to be around.