Do I tell him he's forgetting
Yes, but in a nice way. Something like:
Hello boss, do you remember that you approved me a sick leave on ... (date). I see that it does not appear in the system, and I worry that ... (reason; e.g. you will not have enough vacation days) Will you please verify and eventually enter the info again?
You might continue:
Maybe it would be helpful if we could enter the data ourselves, to make sure that it is there, and you will only need to verify and approve it.
With the above approach, you help your boss save face. The idea presented is that the system is bad, not the memory of the boss. Moreover, you get to be proactive, and try to release him from the work of introducing the data into the system.
Of course, the boss might reject all your requests, but his "anger" will be kept at a minimum.
Imagine the following going on:
Hey boss, it seems that you forgot again to enter the data into the system. Are you going to do it any time soon?
On the other hand, you always have the option do not protect your interests, to give up your benefits - and tell nothing to the boss. If you can live with it, this will work as well.
Should I ask my Boss to add my Previous casual leaves?
Well, it depends on how long ago was previous. It is best to keep an eye on the system regularly, and report any missing information as soon as you find it missing. If there are more than two months, the situation might get trickier to fix, depending on a number of factors (including national laws, company rules and regulations...).