Looking for some advise

We have a 7.5 month old. Since the 4month sleep regression sleep had been abit up and down but recently I felt we were getting somewhere up until a few nights ago. He usually goes to sleep around 7.30pm has a soild 2-3hrs wakes for a feed, and 2-3hrs then another feed, 2-3hrs then a quick cuddle then around 5am ish hes in with me. Although hes waking every 2-3hrs he drops back off for another few hours up until a few nights ago. For the past few nights he has gone to sleep around 7.30pm slept for a solid 2-3hrs had his feed and fell straight back asleep. Hes stirred as weve put him in his cot but then drifted back off for about 45m-1hr then he wakes. We give him a few min to see if he settles, we then try to settle in his cot but then we pick him up, he falls back asleep very quickly, we put him back in his cot and it starts again. Have you any idea why this could be happening? Nothing has changed with his naps (3nap)/feed routine, he has pretty much the same amount of day sleep consistently. Could it be his teeth? We arent sure whether hes ready to drop the third nap, so we are lengthening his awake windows in preparation for dropping it.