I've flown all 3 airlines. I would choose KLM but unfortunately people with US addresses cannot have KLM membership.

With that in mind, if your son is going to be stationed in the US, it really depends on WHERE he does most of his flying. If its on the left coast then neither CO nor NW will do much good.

In general the CO onepass program has had a lot of disgruntled flyers lately. NW on the other hand seems to being a good job in pleasing its customers.