Hi,
the tabasco sauce is a good idea, but it didn't work on me when I was a child, I am 28 years old ( and still suck my thumb when I am tired) and started sucking my thumb when I was 3 1/2 years old, my parents have literally tried everything. They spent so much time telling me to get my thumb out of my mouth that I think it caused me to suck on it more. If you can't get the tabasco sauce or the nasty tasting polish to work, give your son a couple of weeks where you don't draw attention to the fact that he is sucking his thumb and he might stop on his own. When my kids started I just let them be and they stopped on their own. Good Luck!!