You can also boil some white potatoes and mash with soymilk to a runny consistency. Also runny oatmeal will work but do not feed cream of wheat as it will harden in his tummy. If he stops the behavior you will need to get some real handfeeding formula and continue to give him comfort feedings until he weans himself. The worse thing you can do is not feed a begging baby, it set's them up for a life time of insecurity. Your breeder should have given you a baggie full of handfeeding formula as babies tend to regress in a new place. You can call the breeder and see if you can get some to tide you over until you can buy some of your own.
I also agree with everybody else, if the behavior does not stop, to the vet he goes ASAP.
Ginger