We left the JWs when my oldest child was around 8 years old. I was the first one who wanted to bail - mostly because of the horrible, unloving way the JWs treated me while I was suffering from postpartum depression, and partly because I could not reconcile what I had started reading in the bible with what JWs were teaching. After about six months of personal research, and dropping breadcrumbs of information for Mr Scully, he was ready to leave too. Our 8 year old never asked us why we were not going to meetings anymore, but he did ask "If you aren't going to meetings, can I get a ride with Brother X?"
That was when we realized that he was more indoctrinated than we thought - considering our not even half-assed attempts at Family Study™ and our less-than-stellar Field Service™ records. We sat down with him and asked him if he enjoyed being a JW. If he understood what he was learning at the KH. He didn't like it and didn't understand it. He felt embarrassed that he felt he could only be friends with other JW kids, and he felt bad that he missed out on fun stuff at school. So we told him that we had been thinking about things a lot, and doing a lot of reading and felt that we had made a mistake and didn't want to be JWs anymore. We told him that parents want what's best for their children, and we were JWs because we thought it was the right thing to do. When we got better information, we realized we needed to stop being JWs.
He was actually thrilled that he could have friends because he liked them as people, not because they were from a JW family. He was excited about having a birthday party, and Easter and Christmas, and about being free to choose whatever job he wanted when he grew up. From the time he could talk, he always wanted to be a policeman, and it killed me to tell him that he wouldn't be able to do that when we were JWs. He's all grown up now... and he's training to be a police officer. He doesn't remember being 2 years old and telling me he wanted to be a policeman when he grew up. But I do... and I can't tell you the good it does for my heart to know that he's doing something he's always wanted to do.
I can't tell you what exactly to tell him, but it's probably a good idea at this point to not create friction between him and his mom. Maybe something along the lines of being able to choose your own path in life, what is going to be best for you, and the most beneficial for your family. And it doesn't hurt to throw in some humility and say that sometimes grown ups have really hard decisions to make and you're just trying to make the best choice you can for everyone. It doesn't throw his mom under the bus, it doesn't throw the belief system under the bus, and it doesn't force him to choose sides.
If your wife does have postpartum depression, and you want to talk about it, feel free to PM me. Been there, done that. Truly, the JWs were the most hateful, unhelpful, unloving bunch of ignorant jackasses to me while I was suffering from this horrible condition. They were the ones that drove me away from the religion.