I had the impression he wasn't talking about sucking his thumb.
After that it might be only a matter of time before he began to suck his thumb.
I say, let your son suck his thumb in peace.
I saw you at the top of the stairs, sucking your thumb!
Should a child still suck her thumb when she is ready for preschool?
I made him suck his thumb as though he were a baby.
It was all he could do not to suck his thumb.
Near her stood a small boy in very short pants, sucking his thumb.
She stood in the middle of the floor, sucking her thumb and shaking all over.
You could end up sucking your thumb for two, three years.