How can I help my dog overcome fear or aggression towards other dogs?

Fear reactivity and aggression are two different things, but in either case, you should stay away from dog parks. For a dog having trouble dealing with his emotions, a dog park is a scary and overwhelming place with lots of triggering sounds and smells. Take quiet walks instead and reward your dog for not barking when another dog passes by (and ignore and keep walking if he does). If the problem is severe, consider reaching out to a trainer for help.