To do or not to do Paternity test

My husband has 3 sons who are the best kids I've ever met. They are his world. He and his ex are divorced because she couldn't keep her pants on. His oldest looks just like him and so does his youngest. The middle one... nothing like him.
He is now 15, and the other day my mom asked, why doesn't he look like his brothers.. (it wasn't a negative thing, just observation). It made me think... so I asked my husband why he didn't look like him. Looking at his other 2, his genes are obviously pretty strong because they look like younger versions of himself. Well, that got him thinking because he was serving in Iraq the same year his middle son was born. When he checked the dates, he saw that he didn't get home until after the beginning of March and his son was born late October. He asked me what should he do. I asked him if he wanted to know and he said it wouldn't change anything. He would always love him like his own. He also thinks it would destroy his child which has already been through enough because of the crazy mother. I feel like he is scared to know because even though it wouldn't change a thing when it comes to his love for his son, it would change the fact he was his father, biologically. If he has a question, or knows there's a good possibility he's not his father, would he be hurting his son more by not doing the test or doing one? My thoughts are to get a sample while he's sleeping so if he somehow is his father, his child would never know there was question. But if he didn't do it, would his son resent him more down the road if he found out (maybe decides to do one of the dna tests like ancestry) and realizes that isn't his dad yet his dad kept quiet? Do you think his son deserves to know (maybe not at 15) so he has the chance to meet his actual father (if its not my husband). Do you think IF it is another man's dna, he deserves to know he has a son? This is such an eff up situation and I really hope someone who has been in the kid's situation runs across this and can share their experience and thoughts. But any advice would be nice. Maybe someone can share something we haven't thought of from an objective perspective. Thanks y'all!