Here's a page that describes body condition scoring. Http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/agdex4830
I find it difficult to be exact without actually feeling the fat along ribs, neck, etc.., but I would say your horse is probably a 4.5-5. He looks great, especially in the second picture.
Signs of a too-thin horse are prominent ribs, hip bones sticking out, veterbrae sticking up. I think anything 3.5 or under is considered too thin (generally speaking). Seven is getting too heavy (though there are exceptions there).