Yes, OP, it is possible that he might improve. The trouble with this virus is that it ebbs and flows, it doesn't seem to follow a standard pattern.
They things that would reassure me are that, he's eating and drinking, he looks much better, the paramedics have been and didn't appear too concerned, his sats are generally good.
All you can do is what you are doing - which is amazing by the way - lots of rest, food and drink little and often, and monitoring his sats, temp, etc. If he goes downhill again, don't hesitate to contact help again. If he's not already on 6 hourly paracetamol, I would add that - regardless of how he's feeling. Hopefully the antibiotics have kicked in, but they can make you feel a bit off generally.
One thing I would say, is that if he does have a better day, don't let him do too much, he needs lots of rest. The immune system needs to overcome the virus, and if we overload it by doing too much, it weakens it. It's got enough to do beating the virus.
