I think your jack russell is just nervous, probably about going out, its a big world out there for a small young puppy. Have you tried walking him on the lead in the garden ?
Also try to coax him by holding treats in your hand, smelly ones that he can follow and eventually take from your hand, obviously it will mean bending down !!
Our TT was nervous about going out at first until we distracted her from the noises and scary things outside by offering treats every few yards !!.
Now she has gained her confidence and instead of being nervous about things, barks at everything, so now we have another problem !!

Post a pic if you can would love to see it. !