Thanks for your reply
Yes I'm used to using a long line with him. That's how I did the "this way" training. I also still use it if we are somewhere like an open beach where there are many distractions, other dogs, to attract his attention but he can still run around.
He is most definitely still in adolescence stage. My previous schnauzer loved other dogs too but not to the same excitement level as Jasper does. He became "sensible" in realising he got to say hello by staying calm a week before his 2nd birthday. It was weird it was as though he woke up one morning and lightbulb got turned on. I think Jasper will take longer than that, he loves life to the full.
As you say I think the best course of action going forward is keeping him safe on lead or longline and working on more focus training while out. Hopefully when his hormones calm down he will be allowed off lead again.