Dog training help!!! (20 characters)?

Okay so i offered to train my friends dog, I am fairly good at dog training, (I have fully trained around 10 dogs in my life... I am only 15 and definatly not a pro) Anyway i am not the type of person to give up so i am trying EVERYTHING! Okay so lets name his problems! He is definatly a biter, he will bite you if constantly, I have tried shutting his mouth and holding it, pushing under his tongue, holding his mouth opened, snapping his leash, yelling ow, ignoring it, spraying him with water, putting bad scents and tastes on my skin, but everything i do he just come back and bites you as soon as you are done! I need help with that for sure! Also barking! he will bark none stop, they have tried a bark collar, i have tried spray bottle, leash snapping, yelling quiet, trying to distract him, cloling his mouth... nothing works with this dog! Need help with that also! He is doing pretty good with everything else, he is house trained, knows come, sit, paw... I am working on lay down, perfecting a sit, walking on a leash ect... What should i teach him next? Also every dog i have ever trained i have also trained them a bit of agility to burn energy and just for fun you know? but i have not very much expiirience with this breed, but love a challenge.. Would this breed be okay for jumping? Rotti's are pretty stalky, and he looks mostly like one maybe a bit thinner, would this hurt his joints? Also i already know about not starting them until they are 6 months, and only doing it once a week until they are 1 year. Thanks ALOT!!!