it is hard to say if you gave up too soon without witnessing you and your dog in action, or without knowing all there is to know about your baby. (traits, personality, etc)....But dont worry!!! If you find that he is starting to stray again, just re-tether and do some refresher lessons. there is nothing wrong with that.
also if he's lost motivation, it may just be because he's bored with the lessons, etc.. i dont know how you practice at home, or how you respond to him..i try to stay away from using treats (unless its absolutely necessary to make way with a dog) ...you are right, you cant just drop and give a tummy rub in the middle of the street!
but you can get superexcited and bouncy(not too bouncy if this will lead them to jump on you)
but do you use treats at home when you are training? i once had a great dane in class who all of a sudden 'fainted' when he had to heel. this huge dog, would just lay on his side.. knowing he was way too big for his owner to pull a long a bit..treats didnt work, all of us getting excited and saying 'lets go' didnt work... it was funny(well funny now, but not then..lol) for this dog, the only motivation that worked was his favorite toy!! you may want to try that for at home --instead of using treats, use lots excited praise and reward with his favorite toy... if he starts to associate training as FUN, then possibly when you are away from home, he wont think 'oh no not again', each time you go into 'training mode'...
again, these are just suggestions and not a diagnosis for your baby. its hard to know everything you need to know about a dogs behaviour and owner interaction from a messageboard!! but they sure are fun!>
(quick question: you mentioned he loves tummy rubs.. does he go into 'tummy rub' mode during training? like when you tell him to 'down'? or when you want him to do something, he sorta does it, then goes straight into tummy rub mode? just wondering =) )
GOOD LUCK AND HAVE FUN!!!!