Mergers do come with some job eliminations so it is great that you two are cognizant of this.
Any "lay-off" usually comes with a severance package so, in the not very likely event that your husband does get laid off, he will possibly continue to have an income for a while. He could also qualify for unemployment?
If he does indeed lose his job, you may be able to bring down your expenses such as transportation, lunches, etc.
And as a web developer he would, hopefully, find a new job rather quickly.
So, I wouldn't lose too much sleep over this merger. Who knows, maybe the new company will take a look at his salary and say "we need to level you up; you have been grossly underpaid all these years." :)