Good for you for recognizing and acknowledging that his fear is genuine and not just a way to get attention (though kids are good at that too!). Just hold him whenever he wants it -- it only does him good. With older children you can help them work through the specifics of their fears but with a child this young, talking too much about it can reinforce the fear, I found. Do ask what scared him but don't try to go into too much detail or talk him out of being afraid; just say something like, "I know, you're right, that doorbell is SO loud, isn't it? I jumped too. Let's cuddle." And then distract him with a book or toy. Also, be sure other adults or caregivers don't mock him for being afraid, as in "A big boy wouldn't be so scared!" etc. I've seen this happen and it makes the child feel so much worse.
This is a totally normal thing and could go on for a while, and please be aware that as time goes on he may no longer be able to go to sleep and stay asleep just fine like he does now -- nightmare age is upon you too, and nightmares are also normal and also will be a phase that could go on for a while. Like someone else said, be sure that if he does watch any TV (and I think 21 months is too young for TV, but that's us) be sure it's extra-gentle--even Winnie the Pooh's Halloween movie scared my daughter when she was older than your son and we immediately put it away. Kids' minds are very tender little sponges at this age and for a long time to come, and what seems so innocuous to us can be quite scary to them.