Can he be held liable for the resulting losses?
It is possible that your stock broker can be liable, but the feasibility of your recovery depends on very specific facts. Do you have a contract with the stock broker that outlines your relationship? Do you have written evidence of your request for him to buy a significant amount of stock? In any event, I would recommend organizing all documents and information relevant to this situation, and consulting with an attorney individually. A knowledgeable attorney will review the details, and advise you as to your options moving forward.