cj,
I guess for some babies, it's a passing phase. Mine was a major thumb sucker during the first 3 months. Intro the pacifier but he doesn't fancy. Lately, he almost never suck his thumb anymore, preferring to suck/chew on his toys. Sometimes he does it when I'm bfg him and he'd like stuff his thumb absentmindedly into his mouth as well. Like what the #$%@#$@! I'll pull his thumb out and he'll stop.
My niece was sucking her thumb till she was abt 4 yrs. It was like swollen and wet and red and her mom told her that if she continued to suck, worms will grow. Being afraid, she stopped.
Prolly need to wait till he understands things, knows what is disgusting, knows how to be afraid... before you can make him stop.