"I'm putting a lot of stock into leronlimab" is a figure of speech, which Dr. Yo followed up with rephrasing "I have a lot of faith" in the drug leronlimab.
You can't buy stock in the drug, I think he would have said I have stock in Cytodyn if he meant the literal word "stock" I believe. I could be completely wrong in this, but the fact he rephrased and used the word "faith in" after using the phrase "stock in" the drug.

Thats my interpretation of what he said, but I could see how that would be interpreted as him investing in the company.