The standard method is for the player to pick items as if he were a brand new level 9 character: a level 10 item, a level 9 item, a level 8 item, and gold equal to the value of a level 8 item, as JohnP linked on Zachiel's answer.
The wealth difference of this method versus his character who started at level 1 versus the characters who started at level 5 depends on how generous you are as a DM, I suppose, but if you are playing by the books should be fairly negligible. The new items method should net him fewer items, but of higher value, so might actually be beneficial to him over persisting his old items.
Total expected net wealth of a level 9 character is 20,495 gp (level 8 is 14,195 gp). It may be worth doing the math on both his and one or two of the other level 8 characters to see where they sit relative to each other, and deciding how to handle it based on that, rather than based on suggestions-in-a-vacuum based on assumptions that may prove inaccurate.