Yes. Ultimate Finish is flax-based. The omega-3 fatty acids in plant-based sources have to be converted by the body in order to be used, and the conversion is not efficient; whereas the ones in marine sources already in the form the body uses (DHA and EPA.) Here’s an article- https://pubmed.ncbi.nlm.nih.gov/1825498/ (The article says that there were no vegetarian marine sources but it’s been later identified that seaweed and marine plants also have DHA.) The omega-3 vs. omega-6 ratio is also better in marine sources.
Halving the levothyroxine dose will probably help you out, although given the timing, if he picks up weight, you’ll won’t know until next year if it was because he went out and ate grass or because you adjusted his meds! Isn’t it fun managing seniors with conditions? I think you’re right not to change too much else until June so that you can better identify what you’re doing that moves the needle.