Snakes growth is totally controlled by the amount of food they have access to, & their ability (enough warmth) to digest it. It sounds to me like yours is under a year old, since according to the Smithsonian's Nation Zoo, neonates are usually 15-20", & can grow to 36-40" in their first year [if fed adequately].
I'd continue feeding him/her once a week (to allow adequate time for digestion) but it's really important to know what SIZE mouse you're offering. When you say 'baby mouse', if you're feeding a pinkie or fuzzy, that's way too small. He should easily be eating hoppers (ie. weaned mice or even small adults) at that size (20" long & 1" diameter body) so no wonder he's hungry! Sounds like he needs more food!