He can only pocket veto at the end of the session. If he refuses to sign or veto bills passed earlier, they will become law without his signature.
An automatic veto of a bill that occurs if the president or governor neither signs nor vetoes a bill within ten days of receiving it — as long as the legislature adjourns during that period. If the legislature convenes during that period, the bill will automatically become law. A pocket veto cannot be overridden by the legislature, though the bill can be reintroduced at the next legislative session.