lets make this fun for him...
RTS: warcraft 3+ addon. dont search for a realy cheap one or a sale, its not gonna happen. maybe you can get both in one, but otherwise, yes the game is still in retail for a highish price. but its worth it, its the best RTS ever made, servers are still up, you can find a game without problem and the big mods are all up and you can find games without problems(try dota, battleships, shoppingmaul, all realy great mods, and you just have to join a game to download the map and thats all for modding) and it aged very well.
starcraft+addon same like Warcraft 3, some would argue it is even better, but others like me realy like some of the "new" mechanics of warcaft. the gameplay is still miles ahead of every new rts.
you can get it at retail in a bundle with warcraft 2 and diablo+ expansion. thats a good choice, warcraft 2 hasnt aged so well, but its fine, and dia is still a great game.
age of empires 1+2 are good too, but the gameplay was never realy as good as war/starcrafts but the games still feel good, and there are hd ports.
c&c well you can get all of em for cheap, and the singleplayer is worth it. i would recommend generals as your test of the water, the basegame is the best, and the singleplayer is fun, but doesnt have the same cutscene glory than the old.
total war is a nice series too, and you can get the best 3 for cheap (rome, medieval 2 and shogun 2), the modding for medieval 2 is realy huge
star wars empire at war has one realy great aspect, space battles, they are realy fun, specialy in galaxy conquest.
the rest of the game is meh...
and last but not least paradox games. yes they arent rts, but they fit here best.
europa universalis, crusader kings and hearts of iron are all realy great. i would recommend europa universalis 4, crusader kings 2 and hearts of iron 2 or 3. they are way more strategie games like most other rts(alot of rts are more tactic games). but they are not for everyone. the civ series is a bit like it, but the paradox games are way superior.
so that it for rts. i will recommend fps games later, but atm my round of lol started.
) it's from EA, a very old games , Age of Empire Series, Total War Samurai, Civilization V & All Expansion Pack, etc so many LOl
