It is more of Jr's driving style than the car from the HMS that is winning now. Jimmie can drive a loose car and Jr can not. So putting Jr in Jimmie's car is NOT going to help.
Rick has to send Jr to non Nascar tracks to test test test and help him get a handle on driving the POS COT that is noted to be on the loose side with a very small window of adjustments.
Jr also needs to have a strong willed CC like Pops was and when he has his hissy fit will tell him shut up and drive the thing.