Hi, sorry you've had no replies so far.
With regards the thumb, really I'll be honest and say it's hard to stop them! You could try an alternative, such as a muslin, pacifier or age-appropriate safe lovey and see if he will take to sucking on that instead, but there's not really any good way to prevent them doing it as far as I know. It's natural they look for something to soothe themselves and the sucking instinct is very powerful. If you are really adamant about it perhaps you could use some sort of glove or something securely fastened around the hand (so there's no danger of it coming off and becoming a choke hazard) and see if he will suck on that?
Placing a LO to sleep on their front I'm sure you know isn't recommended before 6 months, though once they can do it for themselves there is no reason to fight hard against it as they have the upper body control to be able to lift and move their faces from the sleeping surface. The advice however is you should still always place LO to sleep on their back, and I always used to turn them onto their backs at this age if they needed resettling. Perhaps that would help? Otherwise to resettle on the tummy I found putting a hand on the back between the shoulder blades or patting in this area often helped x
