I have registered 1 trademark, so that makes me an expert. Did this guy register your exact name, spelling, and spacing, etc.? Because you stated he registered "similar". I'm 90% sure it has to be exact for him to even have merit.
You "might" find it confusingly similar (if you're an idiot). I'm trying to avoid outing the exact details here, but lets just call the two sites "American Widgets" vs "US Widgets", where "Widgets" are the same product/service. All the other potentially conflicting trademarks using the word "Widgets" are for different products/services.
The issue derives from the potential confusion between American and US.
Just for example, if the confusion was between "American Rifles" and "US Rifles", I'm not sure how close to infringement that would be, given that US and American have very strongly related definitions.
If I were to sue him, he has already shot himself in the foot by making written statement that he is personally confused by the similarity.