Right now at this age it's nothing but a Passover habit. Say kids will outgrow at the appropriate age. Need not worry at this age. Reason why they do it? Right now at this age It's kids who are not fed properly do it. Try giving him food foods that he has to chew. No no for junk foods. Give him fruits. Chapatti's. Basically foods that he has to chew on.
Time to worry about this habit will be his age where this will be inappropriate.
At this point it's called oral fixation, googled for you, it's bit worrying. But I believe it's ok at this age.
Thank u very much. It's bit worrying for us but made him understand that if he is hungry then to ask snacks. Hopefully this helps.