It should have a white kitten on the front,
Otherwise, when I was nursing a baby mouse, when he started nibbling on my fingers, that was a sign he was ready to start being weaned onto solid foods. So I took a small piece of bread, soaked it in the formula, and for a while he just sucked the formula out of it, then after a couple days he started eating the bread. After about a week of the bread, I took some carrots, finely chopped it up into a mush into the juices leaked out, and tried him on some carrot. Then I tried him on finely chopped apple.
Make sure the heating pad doesn't make up the entirety of the floor. You should probably part it, 50/50. In case he gets too warm.
If he isn't acting sluggish, I wouldn't worry about it. It's normal for babies to sleep a lot. One of the signs you're doing something right is he'll be more curious to explore, instead of eat. During this stage isn't of feeding right away, I let the baby mouse scurry around on my hand until he worked up a hunger.
Whatever formula you use, make sure it doesn't have BHT/BHA, or ethoxyquin. But KMR has been the only one i've been able to find that doesn't contain any of that.
It sounds like you're doing good, though!