Everyone that owns a share of stock in a company is part owner. Some just own more than others. According to Apple's latest proxy statement he owns 5.5 million shares of the 914 million shares outstanding. So he owns approximately 0.6% of the company.
If he owned more than 50% of the company's outstanding stock he would effectively control the board of directors by being able to pick whoever he wanted. Then he would control the company. Very few publicly traded companies are that way. Most have sold off parts of the company to the public in order to raise cash for the company and make their investment more liquid.