Fist/Thumb sucking?

My 10 week old use to take a pacifier, but now he just wants to suck his fist. I move the fist and insert the py, but he cries and spits it out and goes back to the fist. He sucks it alot. It isn't a feeding cue. He has been sucking his fist all the time for the past few days. It makes him happy, but I would prefer the py since we can take that away when the time comes. Or should I just let him suck his fist, then evenutally thumb?