As Dog is My Witness by Jeffrey Cohen
Aaron Tucker is a freelance writer who lives in New Jersey with his wife Abby, a lawyer, and their two children. Daughter Leah is nine, and her brother Ethan is twelve. Ethan has Asperger syndrome, which is relevant because when the president of an Asperger support group asks Aaron to investigate a murder ostensibly committed by a young man with Asperger's, he feels obligated to agree.
Justin Fowler is 21, though he seems much younger. He is very knowledgeable about guns, and when a local man is killed by an antique replica, the police go to him for information. They don't expect that he will actually have that particular gun in his possession, or that he will confess to the murder, but he does.
Justin's mother Mary and Aaron are convinced that Justin is shielding someone, and Aaron determines to find out who it is.
Enjoyable book, clever writing. At times the humour is a bit forced, but it's hard to tell if this is Cohen's style or Tucker's. This is the only book in the Aaron Tucker series I could find in the local library, and I'm going to have to search further afield for the others.
Next up: Bye-bye, Black Sheep, by Ayelet Waldman