Start with all the buttons covered except for one which would be a highly preferred item such as the computer, let him take a turn on the computer then take the mouse away and help him push the button for more computer. Keep backing off your help pushing the button everytime he asks for a turn. Once he masters this uncover another button which would be unpreferred such as a paper clip. Teach him to distinguish between the two symbols. If he touches the paper clip button then give him the paper clip. Once he has mastered distinguishing between the two, then add uncover another symbol of an object. Four buttons at all times may the level where he remain.