My 5 months old keep on sucking his thumb when he is hungry or to smooth himself to sleep. I am worried if this become a hard ot quit habit. Will it go away when he is older?

No, you don't have to stop your child from sucking her thumb while he/she is still a baby. Babies generally suck their thumb to soothe themselves, which is good, because they need to learn how to rely on their own resources. The ability to regulate or control one's behavior and emotions is an important developmental challenge. Restricting them will interrupt their development. You can read up book by dr sear which he did mention it is normal for kid under two to suck thumb to self smooth themselves. I let both my girls sick their thumb and they outgrown it themselves without any need to introduce dummy or pacifier.