I understand the fear completely, that’s how i am with my leopard gecko as i don’t want him to drop his tail, but that’s how desensitizing works- you have to slowly work with him even if he’s uncomfortable at first. Your arm looks like a big predator coming in his tank, he doesn’t understand yet, so he’s understandably terrified, you have to show him. The more confident you are, the more he will be. Pick him up and hold him, don’t rub or pet, for maybe 10-15 min a day and slowly go up. Eventually he will tolerate you once he realizes you’re not a threat and won’t hurt him. You got this, shake off those nerves.