I would avoid trying to stuff him with a bigger meal, especially if you want him to eat more regularly for you. Let him feel a little hungry...it's natural. Most of our pet snakes eat way more often than they would in the wild, & that's what evolution has designed them for- getting by on sporadic meals, not attending regular banquets.

For all you know, maybe that 38 g. rat gave him a stomach ache? It's not as if he can tell you, but I've heard of BPs refusing to eat certain color rodents after being nipped by one of the same color, so who knows whether he's hesitating because of the size now? I have a geriatric (22 year old) corn snake that's plenty big enough to eat adult mice but he seems to know his digestion is now weaker & will only eat small mice (large hoppers at the most) & he actually prefers fuzzies. Snakes are never "one size fits all"- they make us pay attention to their idiosyncrasies & it certainly keeps it interesting.

And by the way, you do NOT want to offer him food that's "the same size as his girth" but always smaller than that. And definitely not larger than... (I'm not saying they don't do this in the wild when they have no other choice, but many things are different in the wild, including their longevity & the amount of exercise they get.)