A sandwich at 12 weeks? You've got a right one there! I'd start as you mean to go on - and if he's a thief now just think how he's going to be at a couple of years old when he can emulate pinkhalf's boy and nick a ham. No time to lose for training.
I'm assuming you're feeding him enough but you should just check that. (He might simply be a greedy guts thief and there's nothing you can do about that - but it's just possible he's a bit hungry.)
You're in it for the long haul, I'm afraid. When he jumps up, take him down - not nastily but not gently either. Firmly. Saying a firm NO! If he goes up again, do it again and take him straight outside to the hall, say, shut him outside the kitchen door and shut your ears to the dreadful pleading. Only let him back in when it stops.
And, watch your voice. No use saying 'No' in an indulgent 'Oh you naughty gorgeous boy' sort of tone. Reserve your firm 'Not Pleased' tone for him, mean it and say it loudly. And Repeat. And Repeat. And.......
Make sure also that the whole family are on side with the training. You might find that you have people in the camp (DCs can be bad but grown ups can be naughty as well) who would be only too happy to subvert discipline by sneaking table scraps to him - just for the joy of seeing the appreciation and not for any evil intent.
That's the only advice I can give. It's worked for me, for a number of cats.
What sort of kitten care things are you worried about? 