I can't say that I have any experience except with housebreaking a dog. In this case, you never want them to become used to soiling their own space. This is exactly what one does when one uses diapers. So, it does not seem so strange that he wants to pee in his diaper because that is where he has been trained to go, against his own instincts. We are getting ready to enter into this venture (due in Oct) and have decided to do elimination communication. My mother said I was pretty much trained by 6 months. She did all of this without knowing that there was a big fancy name for it.
I would do a web search for elimination communication. There are a couple of books. I just bought my 2nd one tonight (the DiaperFree Baby by Christine Gross-Loh and it talks about ECing toddler as well as babies). The other book I got is Diaper Free-the Gentle Wisdom of Natural Infant Hygiene by Ingrid Bauer. There are also a lot of websites and support groups.
I would seriously think about ECing for your 6 month old especially if you are a SAHM, if not perhaps part-time just when you are at home. It is not too late. It might be a good thing for both of your kids to do this at the same time and perhaps avoid many of the same struggles that you are having another 2 years down the road. If the 6 month old is going on a potty, then the 2 year old might take an interest. With luck you could have them both potty trained.
HTH and Best wishes