RSV question

So my 4 month old has RSV. He was diagnosed two nights ago at the ER. I made a post because he wasn’t admitted and I was confused by that, especially since he was premature.

Anyway...his doctor thinks we’re on day 3, possibly day 4 and he expects it to get worse before it gets better. My husband and I got an Owlet sock because I wouldn’t allow myself to fall asleep when he’s struggling to breathe. I watched him literally all night because I would never forgive myself if he had a major problem and I slept through it.

My question is this... what oxygen level is “too low” and would warrant another trip to the ER in your opinion? He wasn’t admitted two days ago because his blood oxygen was staying around 96, with a few drops to 90. Earlier today it was hanging at 94. Now tonight it’s been staying at 90-91 and dropped to 86 but only for a minute or two. It occasionally goes up to 95 again but for the most part is staying around 90/91.