I have to say that punishing doesn't work. It just makes them more resistant. My son didn't get it and really potty train until this past December when he was 3 years and 4 months old. It really is true that they will do it on their own. Sometimes they just don't really get it.
With my son, he fought us and seemed like he was being resistant but then one day, it was as if someone turned on a switch inside his brain. From them on he got the peeing down. The poo came about a week and a half later.
My advice is to come up with a reward system. Something that both you and your husband as well as his Mother agree to. My cousin used Tootsie Rolls. We used Hershey's kisses. For every successful pee on the potty, he got one treat. For a poo, he got 3. He knew where the candy was and knew what he needed to do to get it. It was a great incentive. I, also, promised him a much coveted toy (the Imaginext Pirate Ship) when he went pee and poo on the potty for 10 days straight without an accident. I bought the toy and let it sit where he could see it but he couldn't play with it until he went on the potty for those 10 straight days without an accident. He was itching to play with it. He would touch it longingly. But he still had accidents. Then one day about 2 weeks after I bought it, he just seemed to suddenly get it. Like someone flipped on a switch inside his tiny brain.
With my daughter, my sister potty trained her in a weekend when she babysat while my husband and I were on a trip. She spent one whole day pretty much in the bathroom. They went on the potty for 10 minutes and then she got to play for 20, then back on the potty for 10 minutes. She got it pretty quick. But when I picked her up on Sunday, she wanted her diaper back on. However, after 30 hours of no accidents and no diapers, I refused. It was a little bit of a struggle for 2 days but I stuck to it. After that an accident was rare. My sister, also, made a potty book with her. They recorded each time she tried and whether or not she was successful. If she was successful, she got to put stickers in the book.
Here is the main thing, though. You only have him 50 percent of the time. You have to coordinate it with his mother. If you are not all on the same page, then it isn't going to work. If you make him try and she doesn't, then by the time you get him back, you have to start all over again. So think of the things you could do and then talk to her to see what will work for both of you so that you are working together not setting each other back.
Most important of all, try to be patient. Sometimes it seems like they are being obstinate but they just might not be getting it yet. They may be trying but until they connect that feeling with pottying, then they won't get it. That seems to take a little longer with boys than girls. Also, with boys, the two don't come together. And aiming can be another big obstacle.
Hope this helps and best of luck in your endeavor.