Jack Russell's love to jump and this is often a common complaint among owners. You are on the right track by ignoring him. You must be consistent though. It's not enough just to turn your back, you must completely walk away with no eye contact, no voice, no attention whatsoever. And then when he stops jumping (all four paws are on the ground), you must immediately praise him. It is important to be very calm and gentle in delivering praise so that you don't excite him and have him start jumping again. Teaching him the command "sit" is another technique. When he is strong in this skill, you can have him do this when he starts jumping (making sure you praise him when he sits).
If he jumps up while you are on the sofa, then a firm "off" would be ok in that situation.
It's important to break this habit now while he is still young. Get to work and be sure to report back how it is going!
