Double duty - xc/dh

I just started my son out on some DH, he's 8 and loves it. I have an XC setup on my Mojo and will have to wait till next year to buy a DH rig.

I was thinking about buying a 2nd wheelset and try and make it through the summer. I have SLR's now and was looking at some Deetrax with 2.35 kenda/maxxis tires. Any other suggestions would be appreciated.

Also, I have a medium tuned RP23, 1 1/2 years on it. Should I go ahead and have it pushed to a lower compression now or wait till the end of the year?