I have some 4.5 month old babies who are on medium adult mice and they do just fine. They defecate and shed regularly, and are around 260-300g. They got started on medium sized hoppers, then moved up to small adult mice after 2-3 feeds.

Your snake is not getting enough nutrition, which will lead to stunted growth. It is not a healthy situation..

These people are very experienced and KNOW what they are talking about, trust what they say.