I think JP knew he had to move the body, as the search of his home revealed Topaz Mntn to be where the body was. It may have taken investigators time to establish this (such as going through Steven Powell's journal or computer.) His dad probably pressured him to go. The idea of bringing another person besides him and his dad into this big secret is too risky for all involved IMO. No one would want to do that job anyway, other than the ones with blood on their hands already.
If I try to think like JP, I would have fabricated an identical camping trip with the boys starting late at night, to also give the impression this is something I do on occasion. I would have then driven down to Topaz Mntn, being very careful I wasn't being followed. Along the way there, I would have dug up a new spot (this takes several hours) that was ready to go upon my return. This would have been at least an hour away from where the body was, preferably more. Then I would dig up the body as quickly as possible and get the heck out of there. I would then take it to the new burial site, cover it with dirt, and again get the heck away from the area.
That's a lot of open territory to search; their best bet is to interrogate and try to break one of them. Also, talk to the boys and see what they can tell them. Additionally, at a press conference I would request information from anyone who saw them in the area from WA to Topaz Mntn between August 27, 2011 and about September 1, 2011 to please contact authorities.
Hopefully DNA will be found at the former burial site they have been looking at. I would think that's very likely. That will nail him.
MOO.