Hey, Iv had a couple of orphaned foals, I got one at 4days old, same as this one you got, Il say one thing first you'v got a full time job ahead! It can be very exchusting and expensive! ... Is the owner buying the milk replacer for you, over here its £55 a tub, would get about 8days out of that.
You should get your vet out to give him the once over, our boy got a we injection to give him a vitamin boost, I had to sleep in the box for the first few nights as well because he was so lonely and depressed.
We had the same problem with feeding, I know it sounds curel but the best thing you can do is let him go a 10-20mins over the time he should be fed so you know hes hungry, I pierced the hole of the teat on the feeding bottle a bit bigger so it came out easier and I had to squirt a little into his mouth.. wasnt long at catching on! Like the previous post, make sure its nice and warm, a bit on your arm to make sure its not to hot.
It is possible once they have realised its you that bringing the food to switch him onto been bucket fed but to b-able to keep the option of getting a foster mare Id stick to the bottle for the present. Id try looking for a website for oprahned foals/foster mares, even if you (or the owner) have to drive an hour it might be worth it considering 90% of foals feed on the mares milk up until their 6months.. some longer... do you have the time/money/ patience to do this for that long?
I was very lucky that after a month I managed to get a foster mare for this foal.. I suddenley realised what it was like to have a nights sleep again lol! Good luck to you whatever you decide, it is very rewarding work taking in a little orphan, but alot of work. I'd definley make sure the owner is paying any costs.
keep us posted on what happens!