How well do you and your son work together on the other chores that you do together? Do you both enjoy working together? Do you both communicate well? Communication will be much tougher with the extra noise of an engine and hydraulic pump, plus both of you wearing hearing and safety protection.
If this is your first time working on something complicated together, I'd pass on the log splitter and start with something simpler and safer.
Explain everything that is important several times. Plan on taking more frequent breaks when starting out with a new person. If your son gets tired, bored or scared the chances of an accident will skyrocket.