The problem with the diamond patches arises for those that have been in it since Tiger Cubs, as the Tiger Cub diamond badge is in the same position that the Webelos diamond badge goes. I've always instructed my Webelos families when upgrading to the tan shirt to leave their previous diamond patches on the blue shirt, and they'll get the oval Webelos badge when he's earned it.
I point out to the families that they may certainly transfer the badges from the blue shirt to the tan if they wish, but they will have a dilemma to deal with when it comes time to get the Webelos badge, because you'd have to (a) remove the Tiger Cub badge in order to put the diamond Webelos badge on (nearly all the Cubs I've worked with have been in it since Tigers), (b) remove all the diamond badges you just put on to put on a diamond Webelos badge (I don't recommend this, and I only buy diamond badges for those still in the blue uniforms), or © have a blank pocket to begin with and put the oval Webelos badge on.
I would recommend leaving the pocket blank and when he earns the Webelos badge, he'll get an oval one to go there, and when he later earns the Arrow of Light, it'll go beneath the pocket. Much, much easier to deal with. Plus, he'll still have his blue shirt with his previous badges on them as a memento. Now, while you don't fall into this situation, as your son hasn't been in since Tigers, I'd still follow this practice. The tan shirt isn't designed for the diamond Cub Scout badges, and even though the Insignia Guide says you can wear them, it will have overlap off the pocket and will definitely look funny once you get them on.