Prey size depends on snake size. If he is a baby, he should be able to handle a small <15 gram-ish> mouse or a decent size rat fuzzy. You want something that is slightly bigger than the thickest part of his body.. Typically 15% of his body weight. Results will be a noticeable but not huge lump in him. Here is a chart someone gave me to help figure out my older snakes necessary feed. It helped me.
Rat progression by weight and size
Pinkies = 3-8 grams 1.5-2 inches
Fuzzies = 9-19 grams 2-2.5 inches
Pups = 20-29 grams 2.5-3.5 inches
Weanlings = 30-44 grams 3.5-4.5 inches
Small = 45-84 grams 4.5-6 inches
Medium = 85-174 grams 6-8 inches
Large = 175-274 grams 8-9 inches
XL or Jumbo = 275-374 grams 9-11 inches
XXL or Colossal = 375-474 11-13 inches
XXL or Mammoth = 475-600 grams 11-13 inches.