Upvote:2
Multiple entry Schengen visas are specifically meant to offer flexibility for frequent travelers to Europe, and there is no requirement to travel to the country that issued the visa on subsequent trips. The caveat is that the visa holder is still subject to the 90/180 rule - he cannot stay in Europe for more than 90 days cumulatively (i.e., counted over all trips) within any given period of 180 days.
Your husband should thus be able to use his existing Schengen visa to enter the Netherlands, as long as he meets the general requirements for entry (valid reason for travel, adequate finances, sufficient health insurance etc.), and has not used up his 90 days over the last 6 months.