Toy aggression

Simba’s got some toy aggression issues I need to deal with. What started out with just some growling whenever another dog wanted to play with whatever toy Simba was playing with has escalated to full out snarling/barking/charging/fight-about-to-break-out aggressive defensive behaviour.
He and Nala will play with the same toy just fine together. It’s only when another dog comes by and wants to join in. Then he turns all Jekyll and Hyde on me. Going out on walks and meeting other dogs, he’s fine. Throw a toy into the mix and he’s like “it’s all mine and nobody else can play with it”.
I’m always watching out to break anything up and to take the ball away from Simba. The leash goes on and playtime is over for a while for Simba. It doesn’t seem to be having much of an effect though. It’s getting to the point where the dog park experience is getting ruined for me, Nala and the other dogs. Nala doesn’t get as much play time as she used to because I always have to keep an eye on Simba, and it’s no fun for the other dogs who just want to play.
Not sure how or where to train it out of him, because he doesn’t do it at home, and I don’t want to have him going after dogs at the park all the time to correct the behaviour out of him there. I’m going to have to consult with some of our dog trainers to see if they have any suggestions for me.