Your dog sounds a lot like my girl Jag.
I finally started her schutz. training and if nothing else, I don't feel mortified by her behaviour, because they all have working dogs! Not that I needed to, she did great and LOVED it!!
Keep up the great work! Definitely a good idea to get a harness for him, but the good leather ones should be very adjustable and fit him fine as he grows. My girl is very long and lean right now, but the adult one fits her fine.
Keep us posted!