Do you have a thermostat in your room to gage the temp? There are some really good ones. I have just bought Ollie owl actually and it’s a white noise thing. But he turns on when the baby wakes up and has been great for soothing baby to sleep.
Babies don’t have sleep cycles like us at that age. They get them at 4 months hence the 4 month sleep regression. Is he sleeping in his own cot/moses basket and doing this? Cause he might need swaddled or want comfort from being on you. Babies are primal and instinctively feel safest and protected when on primary care giver and therefore sleep better. It’s so so hard though but it could be that? Also get a room thermometer if you don’t already, and just remember they generally need one more layer than you. X
What temp is the room? I find my baby stirs if it's cooler and prefers a warmer room, or could just be that he takes a while to drop off. does he have a stimulating day? Sometimes they can be overtired which keeps them awake. Try covering his eyes with a muslin over your shoulder or your hand so he can't look around and stay stimulated, also gently running finger from forehead to nose or over both eyes and they'll close automatically. Can also try muslin draped over cot at both sides to cover his view and reduce stimulation but don't forget to take away once he's asleep. there's some good videos on tiktok re this xx
In terms of temperature, you can always air on the side of caution and have the room on the cooler end of what's recommended (16/17 degrees) to give you more peace of mind.