I suggest finding an adult mouse the same size as the widest part of the snake. Its also ok to feed multiple mice in one feeding session or spread out throughout the week. So go ahead and finish that pack of feeders if you like. After a few consistent feeds, go ahead and try a rat pup or weaned rat. The important thing here is to get the snake to a healthy weight. Switching prey now can be stressful and be counter productive if he doesn't take it. I would feel more comfortable switching after he bulks up a little more if he's really as under weight as you say he is.
I also don't like the 10-15% rule. Its strictly for babies and juvies. And people seem to forget that.
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