May I make a few recommendations? First, if buying a cello for him to learn with, get the cheapest one you can buy. Yes, the lower quality of construction DOES make a difference in the sound, but for just learning, have him use something cheap. When he is older- about the time he is in 7th grade (to my knowledge, not a lot of Elementary schools have an Orchestra, so Middle school is the first free class regimen he can take), look into getting a more expensive, higher quality instrument. Just make sure that if he DOES join the school orchestra, he keeps his personal instrument at home, as most schools will provide him with an instrument (although you DO have to rent it from the school)- that rental fee is going to be FAR less than having to pay for a new instrument should something happen to his personal one.
As to when you should introduce him to the instrument himself, I recommend waiting until he can hold down a bass string comfortably against a fret board for this, try taking him to a music shop every now and then. Let him try to see how a violin feels on his fingers as he applies pressure to the strings. From there, go with him occasionally until he is comfortable holding down strings on a 6-string bass guitar (if he can do this without incident, he can definitely handle a cello)- guitar center is a good place to do this, and you run less risk of getting on bad terms with the owner of a small music shoppe or a pawn shop.
Make sure you invest in rosin- a semi sticky substance (usually made from tree sap) used to lubricate the strings on the bow. Its not exactly cheap, but if used right it can last a good long while. The rosin is important- without it, the bow runs the risk of hairs breaking and the instruments strings being damaged. Bass/cello strings aren't exactly that cheap. If we were talking bass guitar you could buy some decent nickel would strings for around $10, but because this is orchestral, the strings will generally be more pricey. This goes hand in hand with the bow- its possible to play without it, but it's like trying to play the piano by striking the interior strings with a rubber mallet not the same in any way, and it can be derptrimental to the instrument and the strings (yes, derptrimental)
Buy a tuner. This will help like you wouldn't believe. Strings (for instruments like the guitar, bass, cello, etc) are sensitive to many factors including time, humidity, even air pressure and temperature. That said, its recommended you tune the strings to the perfect pitch at least once a week, if not more, so as to prevent the instrument from giving off a disharmonious sound. Get familiar with how to tune your string instrument of choice and how an ideally tuned instrument should sound.
The biggest thing I can recommend is this- the bridge, where the strings run across (below the fingerboard, need the bottom of the instrument) is incredibly incredibly fragile. Be careful with them, because they can be difficult to get repaired.
Since you have musical experience, most of this should be like Music 101 for you, but I just wanted to cover all my bases. I wish you and your son luck in this if one keeps at it, the cello can be a very delighting, even rewarding instrument to play, and because it translates easily to and from bass (both orchestral and guitar) he can quickly become a multi-instrumentalist.
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I shall keep them all in mind when I do my homework and look arounds for my son. I definitely plan on being active during the whole process as he gets started on things. So, I'm sure my understanding of things will increase with the strings. 
