We Have An 18 Month Old Boy Who Fell Off

we have an 18 month old boy who fell off a dining room chair and hit his head (left forehead). A bump and bruise quickly showed up, but we put ice on it and it appears to be shrinking. our concern is a concussion. could you advise us what to look for. Pupils are not dialated and shortly after the initial crying, he appears to be back to normal.