I fed my 17 month old king mackerel?!?

Big sea fish such as that, shark and tuna have 'higher' levels mercury in them but not enough that eating a bit is bad for anyone

You'd need to be eating it every day really to get the build up of mercury so go ahead and let little one enjoy a range of food, it's excellent to give them a wide choice

5 bite size pieces of mackerel in 17 months of life is not going to give your child mercury poisoining.

The mercury thing means it's better to limit fish like this to once or twice a week, not that it's some kind of poison you need to avoid altogether. So you can stop freaking out now.

I think it should be OK, just don't give him anymore. It might have mercury but he wouldn't have injested much in five very small bites.