The only way this is possible is if Sihanouk does not get sick (and thereby avoids his trip to France, which allowed the 1970 coup to occur), he purges the military (and nips the coup at the bud), overtly makes overtures to the US and aggressively assists in fighting North Vietnam (averting the 1970 coup), or as a wildcard he declares war on the US and openly seeks North Vietnamese support.
While avoiding sickness, if possible, is an arbitrary but possible POD, purging the military and overtly making overtures to the US probably won't work (unless he goes ahead and seeks medical help in the US, while concurrently doing a state-side publicity tour. The US might decide not to off him.) I think the best chance to avoid the Khmer Rouge is to have Sihanouk seek North Vietnamese support. He in OTL worked well with the Khmer Rouge. If he gets support from the North Vietnamese, this avoids Pol Pot becoming the defacto head of state, and rather Pol Pot becomes a puppet useful in keeping his military in line. After the US loses Vietnam, Pol Pot (who hated Vietnamese and Chinese for racial reasons) will make his play for power, but the Cambodian military with Vietnamese support is more than enough to keep the Khmer Rouge at bay.
Essentially, the above makes Cambodia a Vietnamese satellite sooner. Such a POD is sufficient to prevent the Khmer Rouge, and it can occur in 1970.
Another possible POD is simply to butterfly Watergate. The US, for all intents and purposes, "won" Vietnam in 1973. If Nixon serves his second term in its entirety, there's no chance the North Vietnamese restart the war. In fact, the next President (perhaps another Republican as Nixon was very popular) will not be a pushover like Carter and the North Vietnamese will stay put. The specter of the US pressing its rights in South Vietnam would likely keep the North from attacking again. All of this means that the power-vacuum in South East Asia post watergate doesn't allow the Khmer Rouge to rise from the ashes.