"Impossible to weigh"??? All you have to do is let him hide in a pillow case & secure with a twist-tie. If you still need more support, you can put the bag into an open
box, & weigh all together. Then after you take him out, weight the bag (pillow case) + box (if used) & subtract that weight from the weight with him on board.
It's very normal for a snake to feel afraid (insecure) outside of their enclosure, but it's easier to have them learn & get used to it while they're smaller. The trouble I had with the large BCI that I had for years was getting her back into her enclosure after she was out for a while...she liked it so much she didn't want to go back. (fun times! a bit of a rodeo)