@Hannah we're all sleeping in my mums front room at the minute, but he has a cot at his dad's, will suggest he move the cot into his bedroom so baby can see/hear him.
When my son stayed at his grandparents we set up the baby monitor there. We can access the baby monitor feed from our phones so I checked every now and again (not because I didn't trust them, just from anxiety of being away from him), maybe you could try that? X
Does baby sleep in a cot in own room? If so - you could get him to get a monitor and have on loud with him in bed. It’s a big step but it sounds like it’s going in the right direction and I hope it all works out for you! Maybe reassure him your phone is always on loud and he can always call you if needed. Take yourself out as you deserve it! Keep yourself busy❤️