1. Do you know about the halachic status of Ensure? Is it more assur or less assur than other Chalav Stam products?
Less for two reasons.
1) Probably not made from liquid milk, and there’s a leniency (not universally accepted) for powdered milk.
2) More of a need than for most people, so R’ Moshe’s recommendation of “ba’al nefesh yachmir” would be less applicable.
This doesn’t address the opinions that don’t agree with the heter of R’ Moshe, or hold that the heter doesn’t apply based on today’s milk supply system, and don’t agree with R’ Tzvi Pesach Frank’s heter of powdered milk (even if indeed it’s made of powdered milk).
It also doesn’t address the issue of treifos caused by DA, while supposedly, those animals are removed from the CY milk supply. I’m told that the gedolei haposkim in E.Y. held that today’s “chalav stam” is actually chalav treifah.
2. Do you know if there other Chalav Yisrael products that do the same job?
No
3.Do you have any suggestions for people who need to gain weight and keep CY?
Milk shakes.
There are pareve shake powders made by NutriMax, and GNC, but I don’t know the caloric content.