When it comes to food it's not about what YOU want them to eat (prey type, how big and how often), it's about what THEY want to eat.
Over the years I have had animals that preferred very small prey less frequently and so long the animal is healthy and eating there is nothing wrong with that.
As far as bowel movement, well there is no schedule I have animals that can go 8 weeks without a bowel movement so there is o worry there.
Here are the important things
Make sure your husbandry is optimum, downsize if necessary and feed in the enclosure.
And keep in mind that if you want him to eat more it does not necessarily mean getting a bigger prey, you can offer two mice next time and two adult mice for an adult male is more than enough.