Toddler sucking in bottom lip

1

Hi parents! My 2.5y/o DS started sucking in his bottom lip about 6 months ago when we got rid of his dummy (probably done so too soon, but alas, here we are). Over the months my partner has constantly commented on our DS's bottom jaw now being set behind his front teeth and he's convinced the lip sucking is causing the issue. Any tips or tricks on how to stop this behaviour? I am convinced it is a comfort (due to no more dummy) as I only really notice he does it when he's tired or anxious/ in a stressful situation.
Any guidance is gratefully received!

Thanks for taking the time to read.