Get away from the big box store and go find a good local bike shop. Diamondback makes great bikes, and I would not hesitate to buy one. Finding a good shop is important, as they will fit you to the proper sized frame, and then further make adjustments to things like stem length, and saddle adjustments that will make the difference between falling in love with cycling or being in pain every time you ride and giving up. I cannot stress this enough, especially given the experience he's already had a peepee's. What an appropriate name given the results. Buying a bicycle is like buying new shoes, it has to fit you, and not the other way around.
I worked in a high end bike shop when I lived in Houston, and that was one of the big perks of buying from a shop, we custom fit each customer on every bike sold, including their choice of pedals and shoes. Also, many shops have a basic fitment, and a comprehensive fitment that may cost a bit extra, but well worth it. And it may take an hour or more to accomplish also.