"Not Excluded" means the test could not prove that he was NOT the father. So it means that he could be the father, but doesn't prove that he is.
i think it means he's possibly the father...not sure, but it sounds like hes a possibility. im confused on what else you said in your question. it shouldnt matter if he was the only one tested. if they have the mothers and the childs they can compare the childs to the fathers and see if the dna matches up. its done through gel electrophoresis. they cut up the dna and run it through an electrical thing that attracts the dna, and the smaller the strand the farther it moves through the gel. therefore dna the baby got from its father should be at the exact same level as its fathers dna