This is a tough one, OP. I hear you. Does he have his own room/space? Maybe a makeover would make a nice gift? Let him pick the colours for his wall? Let him pick out his own sheets/bedding? That could be a really nice surprise, especially if you supplement it with some smaller complimentary things he can open.

Does he like drawing? Crayons, paper, paints, pencils, stickers, things like that are excellent fun and are lovely to open.

As for the wrist lead, have you seen these backpacks? You could easily fill them up with tasty treats he enjoys eating and maybe some small toys? Something like Little People or Liquid Sensory toys?

Does he enjoy sensory activities at school? Sand and water boxes? Gardens? Cooking? There might be some strong areas to explore with him even if he’s not immediately interested. Maybe a kitchen set that you could make no-bake foods with? Peanut butter sandwiches or pre-made cupcakes that you can add toppings to? Does he like babies or other children? Maybe he’d like a baby doll and accessories to help him dress/play with the baby? They make all sorts of lovely things for baby dolls these days.

Maybe he’d love to join a small dance group with peers his age? Does he like to move? Does he prefer to be cozy and snug? He likes swimming, and that sounds like a great way to open him up to other water activities - like buying a blow-up swimming pool, maybe showing him surfers or ocean animals, maybe he’d like to have a small aquarium with fish? Maybe a trip to the aquarium would be an amazing gift for him? I know it’s difficult, but is it possible for the other two kids to go stay with family or a friend for a few hours so that you and your son can be together on an aquarium trip? Then you won’t have to stress about him and the other two all at once, which can be a nice bonding experience.

The other things I can think of are Duplo building blocks; clay, and interactive sets - like the ice cream and burger making set is fun; puppets and a puppet stage; oh! One of these peapod chairs; and, as my last current idea, one of these for his quiet space?