Hun anxiety is awful and I know what it's like to get very worked up over what should be a 'minor' issue related to sleep.

I think this is just his way of self settling. 20-30 minutes is a normal time frame to settle down from fully awake to fully asleep. I agree with a PP to look at what you are thinking while he is settling and to try to challenge those thoughts or otherwise distract yourself until he is asleep. Remind yourself that he is normal, that other mum's (myself included!) would love to have such a consistent sleeper, that winding down after the day is normal, adults need to do it and so do babies. My DS is 22 months and he is still grumbling and moaning himself to sleep and that is on a good night. I guess it's all relative but to me it's a sign that your DS is a good sleeper that you are not in here posting about how to cope with crying, calling out and screaming at bedtimes.

Perhaps read up on sleep, sleep cycles, the process of falling asleep (in terms of brain wave activity etc). Maybe knowing the physiology behind sleep, going to sleep and waking up will help ease your anxiety?