dog will only walk when whole family walks

How long have you had him?

We have only had him 5 weeks.  He is very timid - runs away and hides at the smallest sudden sound.  Can we give him more confidence somehow? He is a beautiful animal.

Have a browse through the Clicker Training section, I think that kind of training is a good confidence builder.
 
Aside from that, just give him time..... it will improve as he trusts you more.

I have a working English Cocker (also named Jack), and I know that both breeds at some point, came from the same litter. To build their confidence you need to get them to do what they were bred to do - jump up to catch objects, retrieve, find a scent (hide and seek). My dog's favorite game to play is when I lay out objects and ask him to bring the one I call. It's very easy to teach a Springer to do it. These sort of games will build his confidence in general, and allow him to direct his energy towards something productive

When me and my husband are both at home, and one of us takes the dog out, he'd often refuse to leave and wait for the other... I don't know if it's a Spaniel thing. We have to drug him sometimes. We taught Jack "Such and such stays" - meaning - "this person is not coming", when I say it he doesn't protest and continues walking with whoever. Maybe you could try that...

Do you know anything about his past? How does he behave when you leave him along? does he have submissive urination?

Your dog is gorgeous!
 
I experienced the same thing when I adopted my dog. She was very timid (probably due to a lack of socialization), especially around men. In fact, she is still nervous around my dad even though he is around every day. She is also easily frightened by sudden, loud sounds. What do you know about your dog's history? How much time does your husband spend with her?
 
Also, I think adjusting is factor here. It can take some dogs up to six months to adjust to their new homes. I second the clicker training -- it's a great confidence builder, and my shy dog is living proof of that. You might want to try out "101 Things to do with a Box" after your dog has learned a couple of things via clicker training, as well as games like "Find It". Much like your dog, I had trouble getting my dog to venture out of the house at first. I even had to carry treats around (I used cut-up hot dogs) at first to get her to go further and further. Perhaps this will work with your dog? After two months or so, walks became her favorite thing -- now, she can hardly contain her excitement when it's time for one!

how about trying this: you in wheelchair leave and go and wait somewhere in sight of the end of the driveway. Husband brings dog, presumably dog will be happy to move from end of driveway to you, and you take your group walk.
Then next time try being slightly further away. So the dog learns that a) he can walk with just one of you, and b) if he starts out with just one of you, he might meet up with the other one. How fun.