You know, Lloyd, this is always the 64 dollar question that we all face when we lose a great dog, and finally decide we can't go further without another one. And all I can recommend for your son is a Pekinese like 44man said, or a daschund like Montana Charlie recommended. I now have a half Llewellyn setter, half "mountain" fice (larger breed). She' 10 mo. old and @ 30 lbs. so will be a medium sized dog. Mine's rather loud sometimes, and insistent on getting what she wants at others, but I can't help but be amused with her, and I've come to really love this boisterous little dog. To watch her run about and prance when she picks up a stick like it was the leprechan's pot of gold is something I just can't not appreciate! She loves to chase squirrels, but hasn't caught one yet because of the nearby trees, but I think she'll definitely get one if and when she can find a young one far enough from a tree. She's amazingly fast! Got her build from her Llewellyn mom, and her muscles and love of the chase from her fice dad. And she's independent to a fault! Won't come when called very well at all, but we're working on that now.
Best advice I've ever been given or received is to just go looking. When you find one that's right, you'll know it. And it might be a pure breed or a total mutt. But you have a really good idea what kind of dog your son needs and can manage and enjoy. Just let that be your guide, and use your inner instincts. These decisions aren't always ones that can be thought out exclusively via the intellect, and often depend on what you bump into. Just have a little faith, and don't make your decision too quickly unless and until something "clicks" for you, and inside, something says "This one" to you.
Again, that's the best info I've ever given or received. If you could mail order one like you think you want, when it got here, you might not "click" with it like you expected. So just keep searching a while until you find the one that "clicks" for you and your son. And your son is probably the best one to decide which one "clicks" with him. Wish I had a better answer, but this is all I've been able to come up with in well over 50 years of lovin' my dawgies.