One, two, three - ugh!

I have a dog who every time I take him out he has to do his business at least three times. The first one is always normal but then following that he has explosive runs, if he hasn't been fed he'll just keep trying to go even if there is nothing left to come out! It doesn't matter if he is on the lead or off it. Obviously it's practically impossible to pick up and it means I have to be careful where I walk him, he can no longer come on the school run for instance. Has anyone got any ideas as to why he does this or what I can do to prevent it? It doesn't matter how many times a day he goes out either. My other two dogs don't have this problem and whether I take them all out together or him on his own it still happens
I haven't asked my vet. He is around 12 months old now. It's been going on ever since he has first been out on walks. He is fed dry complete twice a day, a couple of mug fulls in the morning and a couple of mug fulls in the evening. He was beng fed on another dry food previously and it happened then, too. It happens everywhere we go. It's impossible to train him at home as we don't have a garden. It doesn't matter where we walk, if he is on the lead or off it, how many times a day we go out, if he is alone or with the other two dogs I own, it's always the same. And it is at least three times, sometimes can be more. If ever he has an accident in the house, it's always a normal poop, and the first poop when we are out is always normal too. He is happy, healthy, wormed up to date etc. Look forward to hearing what people have to say about it!
Fizz Cashman, via Dogs Today facebook page