KPANDBP is right, it is usually because of the insurance co. The insurance co. will have medical guidelines with criteria that must be met for continued coverage of physical therapy. You should ask your husband's plan for a copy of the guidelines, so you know what the criteria is (might be on plan website, in provider area), and appeal the decision if your husband is told that PT can no longer be covered for him.
Most plans used to require that you continue to be making progress, and if you reach a point you can no longer make progress, they'd stop covering it, saying it is no longer a benefit for you, and thus not medically necessary. However, that is changing with many plans, and has changed with Medicare. Medicare now allows PT to continue if it will allow you to maintain your functional level and prevent decline, and this is a new and recent change in medicare's rules. Some health plans use the same criteria as medicare for PT. Even if your plan does not, it would be worth filing an appeal since your husband can still benefit from PT even if he is not showing improvement, if it is maintaining his functional level. (If there have been times he wasn't able to go to PT, and he had some decline, you can include this info in appeal letter.)
Be sure to get a copy of the rules for appeals or grievances for your plan, and don't do an appeal over the phone (some plans allow phone appeals). It is important to do it in writing, so your position is clearly stated and you can attach supporting evidence (copies of PT notes, letter from doctor supporting continued coverage of PT).