How to discourage thumb-sucking?

You could try to put guidelines in around times for thumb sucking, such as when he is only in the house, & when you see some progress then only when he is in bed. Each time you see him not sucking his thumb, then you could praise him for that.You could also ask him when you notice that he is sucking his thumb, whether he is aware that he is, as he may be quite unaware of how often he has it in his mouth. This may help him to be more aware of what is occurring for him each time he is needing his thumb