Swallowing issues - dysphagia. Has anyone used a similar product?

There are several products on the market.
I used Thick It
The only "problem" with it is if you have to use a product like Miralax you can not combine Miralax with the thickened liquid as the Miralax will cause the thickened liquid to thin out again. (I know, mind blown first time I tried it. ended up mixing Miralax with soups, cream of wheat and other already thick things.)

I used the product Thick It when my husband needed his drinks thickened the last 22 months of his life, and he nor I ever had an issue with that.
I'm glad you found a product that is working well for your brother.

I think all of these products have their followers, my preference was ThickenUp® Clear but that's largely because it was readily available in local drug stores.