Need advice. Panicking over putting my son to sleep in own room.

So my son is almost 4 months and way too big for the bassinet. We made a decision to put him to sleep in the cot in his bedroom last night. Well, he fussed a little and went to sleep an hour later than usual but we got there. Only as soon as I closed the door (we have cats and need to keep it closed) I started to panic. What if he rolls onto his face and suffocates? What if something happens and we don't hear it? Well, let's just say he ended up back in the bassinet less than an hour later. I don't know what to do. I feel like the only way I'll cope with him sleeping in his room is if we get a baby monitor with a camera and an owelet sock (I saw it online, it monitors babys oxygen levels so, say, if he did manage to almost suffocate we'd be alerted) Both of which we can't afford. Any advice on how to relax a little about this?