Well, I don't know. With such a old machine it may not be worth it to upgrade. Maybe you can get him a new(er) motherboard and cpu for a present?
You could probably upgrade it ok, but since it's such a big gap between revisions it may be painfull. If it had been kept up to date thru the differetn versions it probably would of been ok.
If he uses it a alot I'd just clean up the home directory, then tarball it and copy it to a different machine. Format, install, and then setup a new install and then copy back the home directory and untar it.
watch out for those ~/.filename files and directories that contain user preferences, sometimes newer programs have a hard time dealing with old setups, but most are capable of dealing with it ok.