Recently, I read a message from someone on this forum about purchasing an inexpensive oxygen regulator online for $39.95 (I think). A full-color picture of the regulator was shown in the message. I tried but wasn't able to find the message again. I found this same regulator at Harbor Freight for $29.99 and bought it. The only problem is that the inlet pressure is gauged at 0-4000 psi and the outlet pressure at 0-200 psi. I need some way to convert psi to LPM. I suppose I could just hook it up and turn it up till it matches the rate at which my son hyperventilates, but I don't know if that's wise. Can anybody help?

I don't think there is an easy answer to your request.  The mass flow rate equation needs to know the diameter of the pipe or orfice the gas is flowing through and the velocity of the flow. 
You my be able to connect a flow meter to the regulator (one of those plastic doohickies with the little bead that goes up and down inside to give flow rate).

Just open the valves S L O W L Y to get enough O2.

The flow rate in liters per minute is unimportant. What is important watching the bag. Keep it from collapsing all the way, or staying full at all times. The bag should inflate/deflate with each breath.

Marc

Thanks you guys. I'm going to do as you suggest, Marc; it makes sense. Thanks for the information Bob P, I appreciate your help.

It works perfectly, Marc. Thanks a lot!