I think a severe reaction would probably show itself in the first few minutes or at a push a few hours. However, some food reactions can come around 6 hours later as they present themselves in the form of unusual bowel movements etc. (more common for dairy rather than nuts obviously) Id say give it a few hours and you’ll be ok :)
I’m pretty sure @Joss is correct! If he isn’t having any breathing problems or anything I’m pretty sure y’all are safe! I know it’s scary at first when offering the allergy foods. How old is he?
If this is the first time he's had nuts then he may not have a reaction. Often it's the 2nd, 3rd, 4th, etc time they have a potential allergen when they can have a reaction
@Vanessa I was just about to comment this! Our son Is anaphylactic and it didn’t occur on the first exposure. Make sure you keep hazelnuts in his diet now, first exposure should be a finger tip, then 1/4 teaspoon, 1/2 teaspoon, 1 teaspoon. Increasing each time and once it’s a ‘pass’ try to keep it in their diet 1-2 times a week xx
If it doesn't appear within a couple of hours then it's unlikely to happen, but some kids can have a delayed reaction but that would be on the same day as consumed