According to your description, personally, I would advise you to sell away the D70S body and buy a second-hand D90 body.
Try to convince your son that weight and size is inevitable as he is using a decent DSLR and tell him not to worry as he will get used to it after a couple of outings as I deemed that he is too used to the size and weight of a PnS.
Also, tell him that DSLR were not designed to be heavy for fun. AF motors, better covers and sealing, extra buttons and switches... all is to help him to be a better photographer.
Since your son is using P5100, I presumed that he had the basic knowledge in photography. I think it shouldn't be difficult to convince him right?
Shoot good pictures with a light PnS or shoot even better pictures with a more heavier DSLR? Let him do the choice....