Wrong milk can cause constipation problems,puppy or kitten milk is supposed to be a good formula to use - I have read puppy is closer to rodent milk than kitten.
Watered down evaporated milk, or milk sugar +water can be used in an emargency, but they don't tolerate cows milk and it increases constipation.
Since he is too small for your dropper
You could try using bread liberally soaked in puppy milk, or a paper towel rolled to a point and soaked in milk so he can sup from it,some will sup from a drop suspended from a dropper.
Keep stimmulating genitals after every feed, with a damp cotton ball or a damp cotton bud. Feed hourly during waking hours and two hourly at night. If some night feeds are difficult try leaving bread liberally soaked in milk very close by. They will smell it and may try to suckle from it in your absence.
As a child I recall my fathers gentle persistence payoff on a number of occassions.
If you have a little furry mouse toy, or a scrap of fur fabric this may help to give the little guy a sense of familiarity and comfort.
What a lucky wee thing to have found you. he will either give you much joy in the future, or will pass away in a loving and comfortable place.
My best wishes to you and your wee guy.
Pam