dogs and me

Ok I have three dogs. Oscar the border collie Benny a beagle and Remy a cross JRT.
My 'problem' is Remy his recall is a bit patchy. He is slightly brain damaged, if he were a child you would say he has learning difficulties. We found him after he was dumped in a field.
Most of the time he is good and in the house he is like velcro he sticks to me, but if he sees a prey item ie; duck bird squirrel ect hes off into the wild blue yonder which wouldnt be so bad only its not so wild or big round here and two days ago he got onto a road via a fire access and nearly got run over.
We have tried whistles and clickers he is terrified of them and while he will line up for treats at home once he is out food holds little interest.

I tried a squeeky toy which helped a bit, he came to see and got a special treat but Oscar became obsessed with the toy and whined and nudged me for the rest of the walk.
Has anyone got any ideas except keeping him online for the rest of his life.
Something that I can use even while walking all 3 dogs?