My baby stopped sucking thumb

Hanseneva

One might find this a victory of sorts, but at 4 months my baby was using his thumb to put himself back to sleep and sleeping 9 hours. By 5 months he was sleeping for 12! Yay! But now he has stopped at 7 months and I'm wondering if there is a reason. I fear his sleep is reliant on sucking now so we are back to 3 month old version on my son (waking 1-2 times a night). I understand others have it worse, but after 2 months of sleep it is disorienting and discouraging to have regressed.