Expect to take a lot of criticism here. Reddit loves to assume.
Based on your description, it sounds like four things are in order.
Take Ico for a longish walk before the dog park. Burn the edge off his energy so he's not as reactive to the other dogs' energy/behavior.
Keep up the training so Ico returns to you at your call instantly and regardless of what else is going on at the park. Like most dogs, mine would run over to see what was going on whenever other dogs scrapped, and sometimes she'd get pulled into it. It took all summer but I trained her to return to me without delay at the first call, no matter what was going on elsewhere in the park. We're up to about 90% success; usually failure happens when I react too late and she can't hear me over the noise of the fracas. Mind you, it's also important to keep reinforcing that training.
Control the interactions. First, and if you haven't already, begin training Ico to know that aggressiveness isn't permitted. Period. As soon as he begins to act aggressive, grab him, give him a firm "no! no growling/biting," and put him in a time out either sitting still with you or leaving the park for 10 minutes. Second: teach him alternatives. Teach him to respond to aggressiveness by returning to you so you can run interference, or running laps around the park (if he's fast like mine the other dog will give up). Finally, teach him to simply stay away from Happy! We had a problem dog at my park who loved to play with my dog, but was volatile and had a terrible, irresponsible owner. It took several weeks but I trained my dog to stay away from that dog; and when the other dog came over to her, I asked his owner politely but firmly to call his dog off. The won't fight if they don't interact.
When Happy arrives, you and Ico leave. It sucks. But it's the part you can control. If you can, change your schedule so that you're there when Happy won't be. And if Happy is a problem dog, eventually he's going to get himself banned by seriously hurting another dog. Your job as an owner is to make sure that other dog isn't yours.