I'd be looking into a good reputable dog obedience training school. I used to run puppy classes for many years and the oldest we would take them was about 16 weeks (or sometimes even younger for large breed dogs). Puppy schools are also more about socialisation than obedience, so not really the right place for your little one.

A good obedience school should use only positive reward training, and many will now refuse to work with check-chains. If you can find one that has qualified DELTA trainers then that would be great.

Jack Russell's are extremely active, so you're doing the right thing by taking him for a nice long walk. They also never tire of chasing a ball and can run off a lot of steam simply by throwing a ball in the backyard for them.

The mounting thing is often a dominance behaviour (which is why many female dogs do it too). Try to establish the rank in your house quickly and it may minimise that behaviour. Things you can do is to make sure your family always eat their meals before the dog (in the wild the lowest ranking 'family member' always eats last), try to get everyone in the house to stay higher than the dog (so when playing don't lay on the floor and let the pup climb all over you), and when going in and out of the house make sure that you (or any other family member) goes through the door first. Just a couple of ideas

Best of luck with everything!