Rubbish. You don't need a mass demonstration in the US to be effective. The incident is just a few days old. Organized demonstrations will replace the spontaneous reactions to the news. Europe is fertile ground for the pro-Tommy demonstrations. He is much more well known in Europe, especially in places like Poland, Germany, Austria, and the Netherlands.
Tommy Robinson is a political prisoner. The UK government's ham-handed approach to his arrest and imprisonment will invoke greater reactions in the upcoming weeks as the ban on media coverage is lifted. Stay tuned.
A couple tweets from POTUS would be nice and very effective. He should tell little miss May that until Britain has free speech and frees Tommy Robinson he will not visit there.