I think he will be fine on JWB however Argen Grange is a really good food for sensitive tummies too and if you email them requesting a sample they send out a HUGE bag which lasted my 2 forever! I really recommend it but I would just stick with the JWB for a while longer to see if his tummy settles with it.
Edit: Just to add my BSH will eat everything & anything and hasn't had any problems - each is different he may be OK
