At 99 grams you can keep him on the hopper mice until he's big enough for rats or you can try and switch him to fuzzy rats now. Keep feeding him 10-15% of his body weight per feeding until he is older (most people stop following this rule when the snakes reach adulthood - between 600-800 grams). Feeding him every 5-7 days will allow him to grow without over feeding. Feeding more in a sitting could make him regurgitate and go off-feed.

Rats are more nutritious, and as an adult you'll only need to feed one prey item per feeding, vs feeding several mice to an adult snake. Some also never want to switch. I didn't have trouble switching mine. He had been on a small mouse per 5 days at the breeder and for his first 4 meals with me, to switch him to rats I waited 8 days then he was hungry enough to take the rat without hesitation, has been on them since. Untimely it's up to you what you want to feed.