He is somewhat skinny and could probably use more food. When you say he struggles with the size, how long does it actually take him to get it down? What looks like struggling to us can be perfectly normal for a snake. Also, weighing the feeder and the snake and feeding 10-15% up until they're on small rat size is way easier for me personally than trying to visually determine. Widest part of their body is a reasonable guide, but they can eat up to 1.5x as wide as their body. 1x as wide as their body is a better goal but something a bit bigger isn't really a concern.