Too bad you did not know his exact birth date. Most breeders wait to sell a hatchling usually after it has eaten at least twice for them and has shed. Some are not like this and new owners end up with sick babies. I'm not saying this is the reason but it is a possibility. 100 grams and weight loss on a small hatchling like that is a big deal. What were you feeding the snake? Some breeders feed their hatchlings live, tell the buyer they are on F/T to make the sale as most people prefer F/T eaters. Usually, with a snake that small, after a month of not eating it may have been a good idea to have offered a different food type like ASF ot live or taken him to a vet. Losing 1 or 2 grams a week may not seem like a big deal but that is four grams a month. For hatchling, that would be like you losing 10 pounds a week. It really is a big deal. It would not surprise me if the snake died from starvation and dehydration but the starvation and dehydration could have been caused from some kind of underlying condition that was not visible.

You could take it to a vet if it is really important to you to find out what happened. Sorry about your snake. This kind of stuff happens and is all a part of life. It is never fun to witness it or have to live through it but time heals and you will be okay.