At 400 grams, he should be eating a prey item twice the size of a single mouse. So two mice, or if you can get him to switch, a rat weaner (or crawler).

What's your husbandry like? How are you measuring your temps? Can we get a picture of your enclosure? If you're feeding F/T is it possible it wasn't warmed up enough? I've had snakes strike and constrict but not actually eat it because it took them so long to get to the eating point it cooled down and they no longer considered it prey. Also, soaking isn't necessary. It's stressful and I don't do it unless I find a snake cuddled up with poop and/or covered in blood from exploded food items (gross I know).