They do say the longer you wait for exposure the higher the risk for allergies is, but it doesn’t mean he will 100% be allergic to anything. It’s up to you if you want to introduce it into his diet. My child hadn’t actually tried any shell fish as naturally we just don’t eat it. x
We used Ready Set Food powders to introduce any allergens that we don’t typically have at home. Made it really easy. They do say that it’s better to start them earlier to prevent issues later.
Maybe warn nursery so they know what he hasn’t had so they look out for allergic reactions.
My son was fully plant based from birth up until he was around 3 and has no allergy’s at all. Eats everything now.