HELP!! baby sucking on forearm my son

My son was born premature at 23 weeks. He’s 5 months but his corrected age is 1 month. All day he sucks on his forearm for comfort. He does it to go to sleep. I’ve been noticing he’s been leaving “hickie marks “ on his arm. I have a doctor apt for him tomorrow and I wanna ask what I can do to stop it but I’m afraid they will think I’m hurting him. (Which I’m not, he’s in a very loving home) so I’m asking you guys.... how do I stop him from sucking on his forearm.?