Adding/reinforcing to the advice above…
To maintain interest:
– Watch Ken Burns' television series
– Read a general history of the war. I liked James McPherson's "Battle Cry to Freedom".
If he wants to game but does not want to spend time and money painting and basing figures
– Battle Cry (2nd-hand copy of 1st edition)
– Boardgames (either strategic level that cover the entire war or a lower level that covers specific battles or campaigns)
– Paper flats from junior general
If he wants to game and wants to have his own army of painted figures, your statement that you cannot buy or provide an army suggests that you consider this is not feasible. Nevertheless, I am going to give it a try…
You use the phrase "complete" Union and Confederate" armies as if to imply you must have a minimum number of figures on your army.
The rules you use and the level at which you intend to play determine the minimum for a "complete" army.
Regarding rules, Longstreet by Sam Mustafa has a Grand Campaign system where the "minimum" for either army is never more than 54 stands. Of course, the army can be larger than that, but in effect it means 100 stands (for example 70 infantry, 22 cavalry, 8 artillery) is enough for a "complete" army to play the Grand Campaign.
Regarding level, when I painted my first ACW armies (in 6mm but based on stands for 15mm figures) I used Fire and Fury 1st edition (brigade-level). Instead of fighting whole battles, I would use conversions from regimental-level scenarios that covered part of a battle, or play part of the battle so that the stand requirement was low. For example, instead of the full Stones River game in Fire and Fury's "Great Western Battles" scenario book, I ran just the initial morning attack on McCook's corps with the game ending after 8 turns. This reduced the stand requirement to around 100 on each side (and reduced the playing area to a 6' x 4' table which was the space available).
My point is that a "complete" army can be small and you should not assume you need a large number of figures to play a reasonable game.
If you aim for 3 infantry figures , 1 gun, 2 cavalry figures per stand (as suggested by the Fire & Fury rules), the 100 stand army mentioned above translates to c.200 infantry, 44 cavalry and 8 artillery. Old Glory's 10mm range sells infantry in packs of 100 infantry figures, 30 cavalry, 12 guns.
You could do similar calculations with 1/72 scale and 6mm.