Ocean City can be a source of lots of stimulation, or just a little bit depending on what you do. If you want to limit stimulation I'd avoid the boardwalk after 6pm. The beach is no more or less stimulating than any other beach, so it is generally very relaxing.
If you want to enjoy some fun activities, but avoid the stimulation, noise, and crowds, here are some ideas:
Go to the boardwalk between noon and 3pm. All of the businesses will be open, but the bicycles will be gone and the evening crowds will not have arrived. To be particularly low key, stick to the streets north of 2nd Street. There are fewer businesses here and smaller crowds.
If you want to
Mini Golf, go first thing in the morning. We go then and there are few crowds.
Take a trip to Assateague. The
Visitor's Center is nice and is generally not too crowed. They have a touch tank and other exhibits kids like. You can also drive across to Assateague Island and look for ponies. You can drove a couple of miles to the entrance station to the National Seashore without paying the entrance fee.
Visit
Northside Park or one of OC's
other public parks. They are generally not too crowded (unless there is an event going on at Northside Park) and are great parks. Many have playgrounds that kids love.
Go crabbing or fishing from either a commercial pier or park. Northside Park has a pier, so does the
Isle of Wight recreation area. On public piers you need a
license to fish, but not to crab, but your kids do not. On commercial piers you do not need a license.
For meals either do takeout (most places do offer takeout) or go early before the crowds get there. Mot places don't get busy until after 5pm.
I think you will be fine visiting OC and don't need to be scared. Just stay away from the south end of town after 6pm, and you can really limit the amount of stimulation your son will be exposed to.
Have a great vacation!