At 12 weeks walking for 30 minutes is far too much. You would be better spending 30 minutes in the garden and as soon as he wees or poos praise him like mad. You must stay out with a puppy, they want to be with you and if you put them out and leave them they don't concentrate on what they should be doing. Confine him into a smaller area and if the door has to be closed put papers down so he can wee, if you are getting cross when he does anything he is getting confused, so ignore when he does do anything but make it easier for yourself by making the area smaller. At 12 weeks they are still learning how to behave with people, and their mother and litter mates would give a sharp squeal (ouch!) if they got nipped. This tells the puppy how far they can go without hurting. Always have something ready such as a toy or chew to take their mind off biting you. Again ignore the biting once you have squealed your disapproval, turn you back on the puppy and immediately end the game or put the puppy in its pen or cage with a bone or chews and toys. If you haven't got a pen or cage, then I suggest you invest in one, they are worth their weight in gold and give the pup someone of their own they can sleep and eat in, provided you don't use it as a punishment. Spend your time with him playing using a ragger (rope toy) that he can chew on (they need to chew to loosen their baby teeth to allow their adult teeth through). Give him things like toilet roll middles and empty plastic milk bottles which will keep him amused and can be thrown away after.