Giving baby peanut butter

My son is 7 months old. I just gave him his first bit of peanut butter. How do I monitor him for a reaction? I see people say you give small amounts to lower the risk of a allergy; so when can I believe he doesnt have one? I have severe anxiety and I keep panicking. Food allergies are in my top phobias and I just feel totally unprepared in this situation.