it actually would be detremental to let him cry at his young age.
www.askdrsears.com has many many bits of info about sleeping - and the best thing to remember is that a baby cries for a need, they dont do it to make our lives miserable... they dont understand the difference between a need and a want, and they dont realize that anyone else has needs.
knowing that, just remember to always respond to your baby - dont use the cry it out method - of course, please if you are at the end of your rope - leave the room.
anyway, this is perfectly normal baby behavior. you are lucky that the girl sleeps so well. your son probably just has a different sleep pattern than your daughter. dont be disappointed in him - he just has different needs. dont feel guilty for letting him sleep on his stomach, especially during times when you are right there.
have you tried using a sling? the moby wrap is especially comfortable! find it online! using a sling may seem like theres going to be little chance you get a break, but im sure you would rather your son gets some sleep... and it does allow your hands free to do other things.
again, this is normal, after all, he spent 9 months wrapped up warm in your womb, and thats the most natural feeling - he probably wants to just have that closeness. :D the more you can respond to his needs, the easier he will be able to mature and grow and develop the confidence to sleep alone! :D