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I haven't experienced this but I think Louise's advice is sound.I hope you can find the cause and solve the problem.
It wont have happened "overnight" it will just feel like it. You have actually answered your own question. You have said he is a bou and he is 9 months old.
It would be my guess that until now he has encountered some dogs who have treated him like a puppy and now he is becoming mature himself he doesnt knw how to speak to other dogs. YOU need help as much as him and he needs to be carefully socialised now with the right dogs and in the right environment to put him on track for his adulthood.
I would always advocate "vet check" just in case there are pain related issues but I think this is a case of a young man being ill prepared for growing up. Good luck
I hope your great trainer really knows their stuff....
One of the first questions I would be asking is whether or not he is now going for other entire male dogs, whilst being nice to bitches and neutered males, or whether he is going for everything in sight. If everything in site, I would not see him before a complete vet check had been carried out, and that would include a full structural check, and eye check, just in case there is something developmental going on. Once all that had been cleared, I would consider taking him on for an assessment.