adding on to this, since jack's mother is still around in this au i think they would start with his mom's cooking. his mom would have a soft spot for all the riders and would still see hiccup like a second son. she bakes a whole pie when they ask without questions
i want all the dragon riders ro see him also and realize this is actually their friend without his memories and for them to try to jog his memory with little things without telling him he died to not freak him out (it doesn't work)