PS2 slim having trouble in multiplayer selection

My son has PS2 7000 series. It never functions with second controller. At first we thought that the problem is with the controller so we changed it. But it didn’t work. Then we bought multitap, considering that the problem may be with the slot. But still the issue persists.
For example, if he plays Dragon Ball Z, the option for 1p VS 2p is disabled. In any multiplayer game the option for second player is inactive. By any game I really mean ANY Game. We tried more than 1o different games but no luck.
What could be the problem? Please help.