Fish...that's it. Good ol fish ...tastes like fish. Anything else that even tries to add a fish flavoring will have the essence and trace amounts of fish.
Perhaps you'll let your son choose what he wants to eat and you continue to eat in the manner which suits you best? Is there any particular reason he must eat the same? What can I say...I find it rather offensive for omnivores to tell vegetarians to "eat meat" and for vegetarians to tell omnivores "don't eat meat". The freedom of choice is what it's all about.
If your son really likes eating fish, why change that? No one is forcing you to choose to eat meat, eat fish or become a vegetarian altogether. What if someone took away your veggies and forced meat into your diet? That would be so wrong. Why doesn't he get that choice? Is is because he is a child who has no right to choose? Perhaps when he is older, he will choose to become a vegetarian or maybe even a vegan. But shouldn't that be his choice alone? Perhaps this is an issue of "control" more than anything? Just a thought...