He is a bit of a wiggly guy!
Hello the way we do it at our rescue is we take a long string, longer than you know the snake is, place at tip of head and allow him to slither as you measure with the string.
it wroks well
The string trick really is the best way and gives the best accurate reading possible. What we do with our snakes is place the tip of the string under its mouth hold there with thumb gently holding the snakes head and just drag the string downwards its body allowing it to move in a stroking motion then pinch the string at the tail end put the snake away and place the string that you measured next to a tape measure.
If it matters alot, hold down his head and tail while someone measures
