Yeah, he should probably be eating a bit more. You can try switching him to a single small rat per week. But sometimes if they're used to one particular rodent, they won't switch (or it can be very difficult to finally get them to switch). So if he's stuck on mice, you can feed him 2-3 mice per week...all in the same feeding session...not spread out over the week.
If you're feeding pre-killed for frozen/thawed, then you can put all the mice in at once and see how many he wants to eat. If you're feeding live, put them in one at a time. Wait for him to completely finish the first mouse and start looking around as if hunting for more, then drop the next mouse in.
Hope that helps some!