H2bme, I have 2 sons and I know that at that age they could be changeable and torn about what they liked and wanted and what others wanted them to do. As a father you too probably have mixed feelings about all this in case you may be sending him down the wrong road.
Your Son will naturally have mixed feelings about not wanting to be a JW. and will feel an urgency to do something NOW like all young people do. A week, a month or a year, is a long time when you're 15 and religion is pretty boring. He won't want to hurt his Mom and he will feel the same JW guilt/fear, we were all instilled with, yet he will want out and will want to be on "your side".
I'd say it's important that you let him bring the topic up, rather than push the issue yourself. Don't make this about choosing between you and his Mom. Kids can be black and white about things, thinking they have to be either this or that, especially if he's been raised a JW. I'd probably reassure him that this isn't an emergency situation where he has to change everything right now and that he doesn't have to make any choice at all. I'd encourage him to take a "wait and see" attitude and help him see the "grey" areas in these kind of matters and the value of not burning bridges. I'd help him see that there are some positives that come from sticking with things even when he doesn't want to, and following a moral code and a set of standards has it's benefits as well. I'd point out that the problem here, lies in the inability to think for ones self, speak their mind and learn from others whose ideas may be foreign to them at first. I'd focus on having nice times when we were working and playing together. Be an example to him and show him that non JW's can be good moral honest people too. Teach him life skills he may not have picked up within the Org and never say anything negative about JW's or his Mom or her beliefs. Make this his decision rather that one the two of you have made and make it clear that whether he's a JW or not, he'll still be your Son. He'll thank you for that, down the road.