I say thee NAY! There shall not be such thing as XP but onley the progression to thee skills and arts thou hast achieved!
To increase characteristics there shall be MAGICK! An the true magick I bespake of here be they magick of Creo!
Ok, there is more than enough to gain Characteristics as well as virtues (and flaws) so there is absolutely no need to crack with the HR whip.
Virtues to power.
Some virtues can be gained - like some variant of the gift - if you do the right things. Like gaining an infernal contract can instill you with the Infernal Gift. You can also be introduced to a house mystery, like the Bjornaers Heartbeast. And then there are even more obscure ways, for example through Original Research1A or via Ancient Magic1B or other means as decided by the GM, especially in respect to mundane or physical virtues.
You might deem some seasons dedicated to positioning yourself better in the courts (and gaining experience in this or that skill) fitting to grant some virtues, such as title, or an Adventure just brings it with it. Other virtues, like a mages rank, do have specific requests what needs to be fulfilled and come automatic.
Creo Corpus/Mentem & Muto Corpus
Creo can be used to alter physical (corpus) and mental (mentem) statistics with Magic, but doing so in a semi-permanent way demands a pretty hefty spell. Reference the Tables in the core book!
Level 35: Increase one of an animal’s Characteristics by one point, to no more than one higher than the average score for that kind of animal.2
Level 30: Increase one of a person’s physical Characteristics by one point, to no more than 0.3a
Level 35: Level 35: Increase one of a person’s mental Characteristics by one point, to no more than +1. 4
There is all the guideline you need! No XP involved. There is actually no XP involved in the whole Ars Magica 5E, just experience in certain things, and you can't have experience in being beautiful or strong, you can only warp the body or mind to befit the picture. And keep in mind, that permanencya increases the magnitude of spells: Those levels are for spells that befit the "Personal, Momentary, and Individual"5 spells.
For a longer duration you normally would need to push the magnitude by 4 for Year, netting you at least a Level 55 ritualistic spell (due to beiong a level>=50 spell) to gain a personal +1 on one physical or mental stat for one year - which clearly is not a task easily done and is a full 5 points into the "must be a ritual" area. But boosting the power like this is not even necessary really:
There are explicitly spells under Creo Mentem, that raise a characteristic permanently while being ritualistic at a lower level. The Gift of Reason (CrMe35) explicitly says:
"R: Touch, D: Mom, T: Ind, Ritual
Permanently increases the target’s
Intelligence by 1 point, to no higher than 0.
(Base 30, +1 Touch)3b
It is permanent, and it has to be done as a Rite, and thus also demands 1 point of appropriate Vis per Magnitude6, so a sum of 7 of Creo and Mentem Vis in this case. This adds 7 botch dice - dangerous! - unless you manage to achieve calm conditions. Acquiring and preserving these is not the easiest task, but clearly "easier" than doing a rite that has a 1-year duration.
On the other hand, Muto Corpus can give minor abilities pretty easily:
Level 2: Change someone to give them a minor ability.7
For one year, adding 4 magnitudes makes this a spell of level 10 (3 magnitudes for 5, then 1 for 10), allowing it to be cast easily and spontaneously. However, what counts as a minor ability? The standard example is "Eyes of the cat", granting some sort of low light vision.
Using the level 10 base ("turn a human into a land animal") would allow adding drastic changes: virtues like Large, Small frame are well within the doable here, and 1-year duration would bring it to 30, so not even demanding a ritual spell.
What XP does he want to spend there at all? There are no unbound Experience Points in the system! And how to relate "XP" spent on increasing characteristics or virtues to experience in skills? No, that is not intended in the system, it would break the game. You can gain all those nifty characteristics by casting them upon yourself, while many virtues (and flaws!) are to be gained with roleplaying, research and yet again research!
You might tell him about some of the fundamental ideas behind Ars Magica's progression system:
You don't lift weight to become stronger and inflict more damage in the middle ages, you hone your skills in swordsmanship and fighting in armor to increase the damage you inflict. You might become stronger on the way, you might not, but that's a GM decision, as you also age while training. If you start as a child, then you do mayhaps get stronger, but at the same time, children characters do get different characteristics: a flat out -4 to 0 modifier on all of them8.
You don't become smarter because you study, you know better, thus it is raising your skills! Magic though allows you to alter your Mind and Body, so train your arts if you want to boost yourself.
a - the longest valid duration as of the basic rules unless some parts reference permanency is year5. This is a House Rule suggestion:
The GM might increase the chart by two+ steps. If this is done, I suggest to do this somewhat analog to the Arcane Connections9 table, adding Decade and Permanent/Infinite for +6 magnitudes from momentary.
The GM might at his own discretion add other steps between Decade and Permanent for balancing the rather large steps. Another idea to enforce the absolute rarity of such potent spells, would be to have the two+ extra steps each being worth 2+ magnitudes to step up, turning permanency into a +8+ magnitude adjustment (for a minimum of level 75 to alter Characteristics to +1).
1A - Houses of Hermes: True Lineages, p. 26-37, "Original Research"
1B - Ancient Magic (Preview), p6, "integrating ~"
2 - Core Rules 5th Ed, p 116, "Creo Animal Guidelines"
3a - Core Rules* 5th Ed, p 130, "Creo Corpus Guidelines"
3b - Core Rules* 5th Ed, p 130, "Creo Corpus Spells"
4 - Core Rules 5th Ed, p 148, "Creo Mentem Guidelines"
5 - Core Rules 5th Ed, p 111-115, "Chapter Nine - Spells"; here: p 112
6 - Core Rules 5th Ed, p 81: "Ritual Magic" & p83: "Vis must be used when casting Ritual spells, as noted above. [...] For every pawn of vis used, [...] roll an extra botch die if the casting roll is stress
and comes up a zero.[...] includes [...] Ritual spell."
7 - Core Rules 5th Ed, p 132, "Muto Corpus Guidelines"
8 - Core Rules 5th Ed, p 29, "Starting Character Age"
9 - Core Rules 5th Ed, p 84, "Arcane Connections"