A.C.
I remember getting growing pains and I'm 28! LOL! They are quite real. A more recent memory of it though is through my DD. She's 3.5. She'll do the same thing. Those legs start kickin.... She just can't seem to get comfy. All night long. Then the next day, she's like 6 feet taller. If it's really rough on her I give (and gave her then too) some tylenol. It helps relax the little muscles. Also I would massage her legs and give her a warm bath. Those helped a lot too. Good luck!
...I started my DD on milk at about a year so I can't help with the other question. I'd say though that if you DS is taking it OK & has no tummy trouble from it, why not switch???