We got Dasia when she was a wee 5 wks as well....
If you are worried i would probably take him to the vet just to make sure because he is so young. However if you feel that Ollie is doing fine other then the diarhea i would maybe try some pumpkin PUREE (not the filling) and give about a dime size worth of it to Ollie. This should settle his tummy and also should firm up the stool. If he tolerates the Puree well then maybe you can give him alittle more depending if he needs it.
Maybe the cause of the diarhea may be due to the stress of being removed from his littermates
and being in a new environment. I hope your baby feels better and congratulations on the new addition!
Please keep us updated on how Ollie is doing