Try getting him to play in Ireland in 1066, that is usually forgiving for small mistakes. No vassals, few outside threats for several decades, ability to get Primogeniture early, and few claimants running around. His biggest dangers are likely to be Scotland taking Ulster and mercenaries turing against him if he doesn't watch his money (tell him that he should aim for considerably more than the hiring cost when getting the mercs), as the Irish AI tends to be slow with fabricating and pressing claims (though watch out for the dukes). At the start, his chancellor should be fabricating claims, his marshal training troops (or suppressing rebels if he is about to imprison someone), his steward should collect money, his spymaster should scheme against factions and plots, and his chaplain should convert eventual heretics or butter up the Pope). Also have him watch some recent Let's Play videos, preferably after SoA, as there have been major changes since release.
If he has SI, tell him to keep it turned off if he is planning to play a long game, as handing hordes is not something that should be attempted as a new player. I would also suggest that he plays in regular mode rather than Ironman, as it is easy to make small mistakes that can ruin a game (not checking the type of marriage being offered, surrendering instead of enforcing demands, using the wrong CB, accidentally imprisoning the wrong guy and getting lots of tyranny).
After he has managed to form Ireland (make sure he does not keep all the duchies; I did that mistake when I started playing) and hold the throne past a couple of successions (which are likely the largest crisis he will face unless he gives someone powerful a claim; suggest that marrying his daughters to relatives of the King of Scotland is not a good idea), suggest that he tries something a bit harder. I learned most of the game as Robert of Apulia *way* back (before LoR and factions, when the Shia Jihad for Sicily/Fatimid Invasion of Sicily happened before the First Crusade if the RNG hated you), which can be easy unless the Byzantines come your way (it depends on how Manzikert goes and whether there is a civil war after or not), though you start with landed kinsmen who want your titles. HRE dukes are usually quite safe, but you will be stuck with Gavelkind or Elective for a long time (especially since you want the Kaiser to be weak enough that you still can declare internal war, if possible).
Matilda di Canossa can also work (matri-marry!), if you form Italy and get Primo before dying (or only have one son). Elective here is dangerous as there are a lot of strong voters, including the Pope and several doges. A tip is to form (but *not* join) the Independence and Lower CA factions and keep them from firing for as long as possible (the Kaiser can scheme you out if you are unlucky), as you want a reasonably strong HRE to deter the Italian republics and don't want the Kaiser getting any claims in Italy due to rebels. A Russian duke can also work, though you will not be able to expand a lot while the pagans still have attrition (which can come as a shock to new players, so warn him about this). Even so, you can form Rus within a few generations; especially with Seniority.
Aside from that, he probably needs to know about claims (how to get them and how they are inherited) and inheritance (who gets what under the many combinations he is likely to have to deal with), as well as the dangers of strong vassals or vassals with claims. I would suggest that he ignores buildings entirely until he knows the game well, as you can do reasonably well without them. He is better off saving for mercenaries (make sure that he saves money for successions). He should also avoid non-christian rulers and vassals of the Byzantine Empire due to different mechanics and free duchy revokations (as well as the Greek AI being in love with the "Blind" and "Castrate" buttons).
Also, send him to the wiki:
http://www.ckiiwiki.com/
Finally, tell him that it is a bad idea to start big. Anything with many vassals is going to be a problem, as factions will form quickly; and civil wars tend to be messy even if you know what you are doing. In EUIV, it probably is best to start as one of the larger nations to dissuade foreign threats, but in CKII you will not handle your vassals well without a lot of trial and error, which is hard to learn as one of the big players (as the child-king of France, you have a powerful William the Bastard/Conqueror, the duke of Flanders, and the superduke of Aquitaine ready to seek independence early, a claimant uncle holding a duchy, and the HRE wanting their de jure land from you; and that is not as hard as starting as the Byzantine Empire with the Seljuks, the Fatimids, lots of claimants, BitP, and factions all being issues from the start).
When playing together, try starting close and going for goals that both of you benefit from (becoming neighboring kings in the HRE, playing two of the Jimena brothers and conquering Hispania, playing Denmark and Sweden and supporting each other through the early instability before going for Dominium Maris Baltici once you can handle pagan attrition). Decide who gets what, and try to keep your vassals from taking his stuff through inheritance or outright conquest (you can probably deal with his vassals if they try to invade your lands). Occasionally check what his vassals are plotting, if you notice that he misses plots to fabricate claims on the kingdom or to kill his family, and keep some money saved to either hire mercenaries to help him or gift him if he needs money.
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