Indeed with the OP grandson not being a fan of queuing or large crowds l think we can forget all about public transport.
Brighton & Hove Radio Cabs on 01273-204060 is the way to go.
Other taxi services are available.
Providing queues and crowds aren't too much of an issue this must be the way to go. Otherwise walk through the underpass to Sussex Uni and take a prebooked cab from the designated pick up point in the car park. Should be straightfoward enough.