Thanks.
Unless the plan document specifically states that only full time employees who are making between x and y are covered, and your husband would have to take the pay cut in order to be between x and y, this is not legal. If your husband qualifies as an eligible employee as it is without the pay cut, since you are still within 30 days of the last day of your coverage he MUST be allowed to join the plan.
The problem is, the law that governs the administration of health plans is long, complicated and bulky, and I wouldn't have a clue where to point you to give you something you could point to, to prove it to him and since it's Friday, by the time I could find out it would be just about too late.
I very strongly suggest that you contact the Federal Departmetnt of Labor immediately if not sooner and as them for guidance. There is a 30 day window under which you are eligible to join the plan, and you are perilously close to the end of that. It's after hours now but you will want to call them at the earliest possible moment Monday morning.
If it is financially feasible, you might even want to consider taking the pay cut temporarily just to get on the plan, but continue to file a complaint with the Federal DOL over the employer's violation.
Good luck.