Tomorrow, weather permitting, I'm going to put him outside in a clear container so he can see the outdoors and hear natural sounds. I'll bring him indoors at night. Thankfully, I haven't been handling him much. I haven't even lifted him from the box to feed him, but reached down towards him with the food. I will have to try hard to resist the urge to keep peering inside his box.
I also have the telephone number of a wildlife organisation which I'll 'phone tomorrow to ask their advice. If anyone else has any advice, I would be glad to hear from you.
Update 26th July
I rang the telephone number I had for a wildlife centre only to get an answephone message that they were closed for two weeks due to illness. So I googled for wildlife rehabilitation centres and telephoned an RSPCA one for advice. They gave me a contact number for Durham, who in turn gave me Hilda's phone number! Hilda is the Mrs. Doolittle of the North and lives only a few streets away from us.
Anyway, I explained the situation to Hilda and told her how worried I was about little Jack possibly not being able to be rehabilitated into the wild. She told me that, once they're in the outdoors, they will recognise their own species and soon forget their early encounter with humans and that he would have as good a chance as any other baby bird - possibly better considering the regular feeding he's been getting. I told her what I've been feeding him on and she said that sounded fine, but to reduce his feeds to hourly instead of half-hourly. I have to encourage him to peck at seeds by dropping them in front of him and attracting his attention to the seed. Once he is able to peck food for himself, then that is the time for him to go. Hilda says she'll help me with that. There's a wildlife area at the nearby allotments, away from lots of people, but close to a number of bird feeders, so it will be a relatively safe place to conduct his release when the time comes. I feel a lot better now since speaking to Hilda and, since she has raised and released many young animals and birds, I have faith in her. Once again, why didn't I go to Hilda first!