Some foods tend not to flatten out on a baby's tongue as easily as others. Might be that there's more cellulous in green beans and its not as easily to get off the tongue as fruit purees and sweet potatoes? Some websites say babys don't develop the ability to swallow solids correctly until 4 to 6 months of age... So, your son's mastery of swallowing might be just a touch too young for certain types of foods. I'd hold off on the green beans for a while since he didn't do well with them.
BTW, I don't recall starting green beans at 6 months. Admittedly my recollection might be fuzzy, but I think my DD started on rice cereal just over 4 months, did that for 3 weeks, then my pedi said we could introduce "yellow foods" (apples, pears, bananas). Then we did those at least for 3 weeks (one week for each fruit), and then it was the other types of cereals: rice with fruits, oats, bran (one week for each of those; can't remember when we did wheat cereal). I think we then moved on to "orange foods" (carrots, sweet potatoes) when she was 6-7 months. Then there were a few weeks when I combined 2 of those foods together (like SP and apples, apples and carrots, etc.), just to get her used to mixed flavors. Green foods were introduced when she was a touch older, maybe around 8-9 months. Of course, its possible we were just going really slow because I was paranoid about signs of allergies-?! Or maybe I was just too exhausted to go any faster! LOL