40~50 grams is just fine for his size.
I wouldn't keep feeding him live, or else he might get stuck on live and really not take f/t in the future. He's already up to the size where some males go off feed for a few weeks, or even months. As long as he's of nice chunky body condition and not losing weight, don't worry about it- just offer him a f/t rat once every 7~10 days (or even longer) and wait him out.
If you have a scale, keep track of his weight to make you feel better! Almost all sub-adult~adult ball pythons go off feed and us keepers just have to offer every week or two and wait until they start eating again.
(I really insist to stop feeding live just to make him eat LOL! Having a snake stuck on live food is no fun.)
The other possibility is your temps are off, so re-check your temps.
To make you feel better, my adult male weighing about 1100g went off feed from November~FebruaryI just offered once every 2 weeks until he started taking again.
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