It is wonderful that Seamus is so loving with people. Some times dogs can be anxious with both dogs and people and it is much more difficult to address the situation when there are multiple triggers.
I am glad you are using desensitization. Keep in mind that true desensitization occurs when an animal is not under duress at all. If he is stressed, even in the slightest, repeated exposure to a stress-inducing stimulus is called habituation. In my experience, I have found that desensitization can be slow going at the beginning, but results in faster results long-term.
It is great to use an area with a fence between Seamus and the other dogs in the park as long as the other dogs are relatively calm and not "fence fighting" towards Seamus. Having a barrier allows you to monitor his stress level at a specific distance and gently move him back if he is reactive. Always make sure that Seamus is calm at every stage.
There are some strategies and points to consider when addressing your question about whether Seamus will ever play safely with other dogs.
- Every dog does not want to play with other dogs. I have worked with many dogs that seem ok at the dog park, but show no interest with the other dogs and want to leave as soon as they get there.
- Try and avoid putting a specific timeframe on his performance. Instead, pay attention to keeping him calm during each interaction with other dogs. An interaction happens anytime Seamus sees, hears, or smells another dog.
- Try and make a bit of progress every day. Progress might be as simple as getting one step closer to the dog park.
- There is a lot of value having Seamus remain at a constant distance from the dog park. Don't feel that you have to continuously move him closer to make progress. The goal is to have him be around dogs and remain calm every time.
I like dog parks in theory, but sometimes the dynamics can be ruined by one or two unruly dogs and ignorant dog guardians. As long as the behavior of the other dogs and dog guardians are appropriate it can be a great experience for all. Specifically, I look for the following in a good dog park:
- A clean, well-kept area
- Non-aggressive dogs
- Responsible guardians that clean up after their dogs, pay attention to how their dogs are treating other dogs, and leave if their dog is being unruly
- No dog walkers that bring more than two dogs. I have seen dog walkers rumble into parks with as many as 12-15 dogs per person. There is no way that person can manage all of his/her dogs.