Necropsy is the only way to know for sure, but I personally wouldn't put the money toward it. It sounds like he just had an underlying condition that got the best of him, because your husbandry was on point. I mainly work with garters, and with those large litters you always have a few failure-to-thrive babies. There doesn't seem to be anything wrong with them, and some will even eat a few times, but they'll suddenly die for what appears to be no reason. I'd chalk this up to a failure-to-thrive and put the money toward a new snake.

Just to be safe, it might be a good idea to clean all the cage decor and replace bedding before putting anything new in the enclosure.