Exactly. But if CO is your first pick, start with that. Book him on SkyTeam flights (CO, DL, NW, AF, KL etc) and he'll quickly earn status on CO. (Caveat: some lower fares do not earn full qualifying miles on CO. If he flies full fare or business class this won't affect him, though.)

When CO switches to Star Alliance, possibly next year, you can continue to book on CO, plus other Star Alliance airlines (UA, US, AC, LH etc). His CO status and accumulated miles are pretty much guaranteed even if CO does switch alliances.