Okay!
Ped's first reaction was that it was a stomach bug and wants to remain optimistic that that's all it was. She suggested I keep DS on the Alimentum for another week and then introduce the Enfamil Soy (which he was previously on) again and see if there is reaction, then go from there. He's lost a bit of weight, is only in the 25th percentile for weight for his age which kindof concerns me but he's also very active, she said it wasn't anything to worry about.
So, for now we'll use up the Alimentum, and give the soy a try again next week.
I feel better...I just hope he doesn't have the allergy and that it was a bug.
:-)